Ruby's semblance transforms her into rose petals from the outside in unless she chooses otherwise, starting with blood vessels to prevent bleed out from use. If she completely transforms, which she can choose to do or not, she will die if she is transformed when her aura runs out. Her semblance has a side effect of making her invisible because of the look, so if you see her move, and even if you don't, it's probably her natural speed. She transforms her clothes into rose petals to partially to obscure her movements, and though she can make herself visible while using her semblance on her body, she chooses not to in order to avoid trauma. She uses that trick to beat uncle qrow sometimes. She can control her petals movements, and once they're disconnected from her, they can reattach anywhere on her body, so theoretically she could regrow limbs with it, though we've never had to test it. When she uses it, her aura splits the same way her body does, so each petal has its own amount of aura before it dissolves. That's why she can use her semblance for so long without being affected, and also why she doesn't use it nearly as much as it seems like she does to the student body. When she speeds up more than a non-certified huntsman can track, it's usually because she dissolves her legs to the knee, and uses her petals to hold herself up. She could also manipulate the way her aura separates in such a way as to make the petals sharp, and possibly fuse dust into them to make them more solid, flaming, lighter, faster, sharper, duller, smoother, longer lasting, durable, or larger/smaller.
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Ruby stood up as Ms. Goodwitch chose her to be Pyrrha's next opponent. She might have had to use more of her skill than she displayed previously to win. Oh well. It'll be a good test, since Pyrrha could move metals, like crescent rose, with her mind essentially. Ruby nodded to herself, she'd give it a try the way she normally did, then ramp things up until she won.
Yang turned, "Good Luck, Pyrrha, you're gonna need it!" This, of course, prompted a head smack from Weiss, and disbelieving glances from team JNPR, and Blake. Yang decided to explain to the two teams, minus Pyrrha and ruby, who had already started to make their way down to the ring. "Let's start with the fact that Ruby's semblance isn't actually speed." Yang said, countered by Weiss' "she's always told us that it's speed, why would she do that if it isn't?" Yang shrugged. "She wants to be strong enough that she won't have to use it as anything more than a party trick. There's some real shitty downsides. For one, if she fully activates it and runs out of aura, she'll die. That's the end of that. Hereditary semblance, and it killed her mom. She doesn't let it bother her, but it's still why she doesn't use it a lot. You'll see the rose petals, that's her semblance." Ruby and Pyrrha squared off, and Goodwitch sent the start signal, the arena immediately filling with petals as Ruby runs towards her. "If you can still see Rubes, she's not using her semblance on herself. She can transform anything infused with her aura into rose petals. You'll notice that the petals hide her movements, that's what she wants when she turns her cloak into petals." Weiss looked shocked, finally noticing as ruby appeared out of the petals behind Pyrrha without her cloak, swinging her scythe. "So That's why she cared so much when it got pinned by the feather during initiation! I thought she was just being childish." Yang shook her head. "No, that's not why. She would have damaged it by transforming it, but she knows how to repair clothing. She didn't want to change it, it was a gift from her mom." Blake glanced Yang's way as Pyrrha used her semblance and shield to redirect Ruby's swing. "You keep saying her mother. Aren't you sisters?" Yang laughed. "Do we look related? Don't answer that. We're actually cousins, we grew up together after Uncle Qrow and Mom moved in." Blake nodded. "That makes a lot of things more clear." Yang nodded back, still chuckling. Oh hey, you're gonna wanna watch this. Ruby's getting serious for once." Yang said, suddenly making the two teams of 3 lean forward to watch Ruby stop moving, allowing Pyrrha to close the distance to get a few solid strikes with her spear and shield. Pyrrha stabbed at ruby, and the blade went right through her, shocking Pyrrha, Goodwitch, and the spectating students that weren't Yang. Goodwitch was about to call the match and send Ruby to the medical wing ASAP, but Ruby slapped Pyrrha across the ring less than half a second later. There was still a hole in her chest, which flooded with red rose petals and vanished a moment later. Glynda cleared her throat, "Ahem, um, Ms. Rose, if you'd like to go to the infirmary-" Ruby shook her head, resetting her scythe stance as more than half of her body dissolved into rose petals, leaving her as a torso floating on a cloud of petals, with a scythe floating nearby. Only a few, shaped vaguely like legs stayed with her, as most flew to combat Pyrrha with the scythe. "Ruby's semblance allows her to completely or partially transform anything soaked in her aura into rose petals, which she can control mentally, and she is aware of how long each petal will last and where they are. It's almost as versatile as your glyphs, Weiss, because until she runs out of aura, she can turn her body into enough rose petals to completely obscure an entire kilometer. Of course, she can only split completely into petals for about 30 seconds before she has to start putting herself together again. She doesn't have enough aura to use it in combat, but she can transform her combat clothing, and crescent rose as well. See?" Ruby was manipulating her petals, her scythe turning into petals after Pyrrha starts to use it against her. Ruby vanished into a cloud of rose petals. "Ren looked worried. "I thought you said she couldn't use it in battle?" Yang nodded. "She's not. She just turned the rest of her outfit and the outer layers of skin into petals. Like I said. She would just be gone if she completely used it. You'll notice soon. She's just getting a new weapon right now. Weiss, this is a secret for you." Yang prodded, and Weiss gasped. "How much Dust does she keep on her person?! I don't even have to try to sense it?" Yang nodded, Ren and Pyrrha looking far more surprised than Weiss did, which makes sense given the difference between their ability to sense dust. "Ruby's entire outfit is filled with Dust, Weiss. She trained for years to be able to use her aura on Dust without activating it. It's her signature technique if you force her to stop using crescent rose. Now is when the real fight begins." Yang bragged. "Pyrrha can't use her semblance, and Ruby is totally relying that fact." Ren noted in response. Ruby smiled. She could hear them well in this state. And she agreed. It's a matter of semblance skill vs martial skill. Ruby reached through the connection to her petals to catch pyrrhas shield and transform it into petals as well, then she activated the Dust she had gathered, turning her petal storm into a deadly array of slicing circles that quickly chopped Pyrrhas aura away, even as they were destroyed left and right, reducing Ruby's aura pool, rather than her aura levels, which seemed to stay the same. Soon enough, Pyrrha's hit the red as she attempted to even find ruby in the complete obscurity she was in, and Goodwitch declared Ruby the winner. Ruby coalesced back into a human with clothes and weapons, also handing Pyrrha back her shield, which resulted in Goodwitch noticing that Ruby's aura was almost as low as Pyrrha's, and was in the red, likely for longer than Pyrrha's was, and Goodwitch amended to that effect. "Actually, it was a tie. How did your aura stay so high until after the fight was over, Ms. Rose?" Yang coughed. "Her aura pool was falling instead of her aura amount. It's a side effect of using her semblance on more than her cloak, and it takes a lot out of her after she loses the adrenaline." Yang said, gesturing to Ruby's suddenly unconscious form as it fell onto Pyrrha, who caught her and started off to the infirmary. Nora came up right behind Yang, and loudly proclaimed, "so what would've happened if she ran out of aura there?" Yang shushed her, then said, "Ruby would've been without legs or arms for a while until she had enough aura to activate her semblance on something. She can use it on anything that can hold her aura, which means that it's not impossible for her to use it on a chair or bedsheet, which she'll have to use to rebuild her limbs. Still, we don't know if she could do that if her arm was cut off instead of happening because of her semblance. But until that happens, we won't worry about it." This worried everyone, almost more than the idea that completely using her semblance would kill her in half a minute. Then they registered what exactly was implied. Weiss was the first to realize, as is usual, replying with, "Wait she's done that before? You mean to tell me that she has essentially cut off her own arms?" Yang nodded. "I said before. She knows exactly what's transforming and where it goes. She's turned me into rose petals before too! If my aura runs out while I'm petals I'll just reform. I assume of course, that if her aura runs out before I reform, then I'll probably die, but I'm liable to die before I'm 40 anyways, given my career choice." Of course, casually mentioning your own possible death would worry all of your friends, and Yang was immediately drug off by the other 6 team members.
A few minutes later
Yang was sat in a chair in the RWBY dorm, casually watching the 12 concerned eyes that were looking her direction. "I take it you have questions? I've already Rose to the occasion to answer them all! Or most of them. You'll have to ask Ruby about the rest." Jaune went first. "What do you mean she'll die if she runs out of aura while in her semblance?" Yang shrugged. "Exactly what I said. Her semblance is an on off switch with a gradient scale. Once they're petals, she has to activate her semblance again to turn back. Her aura shields her mind from being shredded when her brain is shredded, so the moment her aura runs out, if anything immediately vital is gone, she'll die before her aura returns and she can change it. She's really fucking good at biology because that almost happened once. She doesn't have improved mind power from her semblance either, so if she forgets where any one thing in her body belongs, she's fucked if that thing is vital. I've gotta say though, it's really weird watching her fix broken bones that way. Like, her arms just kinda flops down and it looks like she has razors for bones-" Jaune cut her off there, "nope. You can stop now." Yang laughed in response. Pyrrha took her turn now. "Why did she choose to become a huntress with such a large handicap? If she could die in a simple spar, why would she even do that?" Yang nodded slightly, "I ask her that a lot. One: she's an adrenaline junkie. She unlocked her own aura before she started school from all her antics. Two: Summer was a huntress, so obviously it could be done. And three: she really wants to help people. She could have helped as a police officer or shop owner, but they wouldn't fulfill the adrenaline rush or legacy, so she chose to be a huntress." Blake decided on a new line of questioning, given the new information. "How did she unlock her aura?" She asked. "Not so funny story actually. Well that's not quite true, the Xiao Long family finds it hilarious now, wasn't that funny at the start. She unlocked her primary semblance and her aura at the same time - a tiny Nevermore had gotten into our house on patch, and dive bombed her. She didn't move at all, and it went right through her chest, right about where Pyrrha's sword went. She had unlocked her aura and semblance in the half second before it reached her heart, and all that we saw was red petals shoot out her back as the thing went through, and the nevermore got stuck in the floorboards. We rushed her to the hospital, and they said that she was perfectly healthy, especially for someone who just unlocked their aura. No stress whatsoever, just an adrenaline high. She really loves being in situations where her death is a possibility, and I legitimately think that she wants to. But that's a conversation for another day, since she won't do it herself."