Kit's grin is either off-putting or amusing depending on who stands across from him. Behind the grin, he is either highly amused and enjoying himself or… highly amused and enjoying himself. Granted, one of the two carries a heavier sort of joy. Joy tinged with muted anger at the messed up chaos that reality often finds itself in.
Every single Jedi knows a bit of Shii-cho. The younglings learn it first since it emphasizes simplicity and fundamentals. Train the legs to move correctly, train the arms to move with the legs, train the wrists to become flexible and loose in a firm sort of way. Learn the angles, learn body targets, learn basic dueling tactics. It's all there.
Kit has refined this simplicity to an unusual degree and has instead turned it into a sort of patterned randomness. Patterned because he knows exactly what he's going to do next. Random because no one else has a clue. It's always different.
So when Obi-wan watches the Nautolan Jedi duel the JK droid for the first time, he doesn't even bother to try and guess Kit's strategy. He's only focused on tracking the green blade that's moving impossibly fast in swift, simple, effective strikes and trying to figure out whether this droid is sentient in some way. Because those tentacles have unexpectedly plunged beneath the dirt and are more than likely scurrying towards a place beneath the Jedi's dancing feet, and how would a droid be capable of trying something so unexpected?
Obi-wan's gut churns when they reappear, stabbing towards Kit's ankles like four angry sand adders. Kit's slight grin actually grows as he briefly retracts his blade to stab down at the offending bits of metal. Obi-wan would have flipped out of range. He's certain his apprentice would have annihilated the tentacles while cursing them out in Huttese. Master Yoda would have flipped out of range as well. Mace wouldn't have let the things get close.
Kit chooses to take a step deeper into the droid's metaphorical (or literal) jaws. Into the belly of the beast, as it were. Towards this strange, cold killer of a droid that's far more dangerous than its predecessors. Apparently, the only way to counter an unpredictable foe is with an equal, or possibly greater, dose of unpredictability.
When the JK ends up stabbing itself to death in a vain (panicked?) attempt to kill Kit, the Jedi lets it succumb with a smile that is now both pleased and thoughtful. As if he'd never been troubled at all.
Which he isn't. Kit lives a simple life. Fluid, adaptable, and determined. "Simplicity has a distinct advantage," he tells whoever asks. "One no longer has to think, and that's when a person can begin to play."
This scene is borrowed from The Cestus Deception (it's a great read!)