He had been worried he would not get the opportunity at all. Bellatrix had been watching him like a hawk. Every time he looked over at her, she was staring at him, practically daring him to walk over there and do it. Every time he didn't she gave him a smug little look. He was going to wipe that look off her face. He was going to get the upper hand. He was going to show her.
When the opportunity presented itself, of course he was going to take it. Flip her skirt up and prove he had been right all along. He was sure he was right, that she was wearing ruffled underwear, but it was important that she knew he knew he was right. She knew now that he was right, he was certain of it, but she could sit there and be smug about the fact that there was no proof he was right.
However, it did not go as smoothly as he had planned. In his mind, he grabbed her skirt and flipped it up, giving everyone in the area a clear view of what she was wearing. She shrieked and ran off and he got away with it completely, but later he'd go up to her and smirk and say: "See, I was right." In reality, he grabbed her skirt but she moved right as he went to flip it, so she felt the tug and spun around as he attempted to throw it up. Which meant her skirt didn't really go anywhere and she was staring at him, while he still had hold of her skirt.
But that didn't mean he was going to give up proving himself right. He was going to have to give up not getting into trouble. Bellatrix's eyes flashed as if she had every intention of burying him out in the garden, but there was a mission to be had. Only thing to do was to proceed. He was going to see what she was wearing underneath that skirt no matter what.
She shrieked and grabbed a hold of her skirt, in order to prevent him, and soon they were both tugging at her skirt. In the tussle, he wasn't sure exactly how it happened, they ended up on the floor. Being bigger than she was gave him the upper hand and soon enough he had her on her back, straddling her legs. He had won. In a defiant move, he successfully pushed her skirt up enough to prove to everyone in the room that he had been correct all along, a gleam in his eye as he announced it.
His triumph ended when Bellatrix's palm connected with his nose.
