Mysterious mentors always have to start their lessons with something bizarre that the student doesn't understand the point of. The important things are to show the student that they don't know everything, that their assumptions about the skill they're about to learn and the path they are about to embark on are completely wrong, and that the mentor is wise and experienced enough to know these things.
Or that the mentor really is just eccentric and half-crazed. Either tradition is good.
Commentary by memnarch (who has not seen the movie)
Ah, a leaf makes much more sense. A good improvised teaching tool. I'm not sure where Luke would have grabbed it from as all the plants seem to have been mosses or lichen, but that's just a small nitpick. Also? Looks like Rey was smart and went with the "sitting" form of balancing on the rock if I'm not mistaken. It's a little hard to tell with the camera that far back though.
So is Rey meant to feel an emotion from this? Pete certainly has "annoyed" down pat if that's the point. Or maybe it's a "feel what's on the other end, or inside, the leaf" kind of thing? That'd make sense as training a Force ability of some kind. Comic-wise, we just were told shed leaves have midi-chlorians in them, so that'd be the reason I get to learn about at least, if true.
Rey: What’s this lesson about? Kicking ass with laser swords?
Luke: You know those things are dangerous, right?
Rey: That’s the point!
Luke: Being badass isn’t all about being badass, you know.
Rey: Now there’s a quote that’ll go down with all the other wise old mentor things.
Luke: Be glad you didn’t have Yoda teaching you.
Luke: Close your eyes. Hold your hand out in front of you.
Luke: I tickle her hand with the leaf I’m holding.
Luke: What do you feel?
Rey: You’re tickling my hand with a leaf?
Luke: No! I mean, well, yes, but what else do you feel?
Rey: I feel this is a waste of time when I could be levelling up my laser sword skill.
Luke: If we started with that, you’d have lost this hand already.