Rachel dances dirty. She gyrates her hips and turns on her toes, moving in all the ways that turn a guy on. She sways and her breasts sway with her, she breathes from deep in her body and draws attention to the sum of her parts.
What Puck does is he watches her. He watches her move and watches her breathe, her chest rising and falling and her pink lips parted like they're some sort of promise.
Sometimes he dances with her and they move together, in sync because she's the person who gets how to do this, how to show love (or not that, signs of affection that are just attraction, lust, whatever) through your body. Even though Puck's normal way is a little, um, raunchier (fuck yeah) and Rachel likes to combine her dancing with step-step-touch and grapevines and jazz hands (Puck hates Broadway dancing, thinks it's for pussies).
Eventually she's ony dancing with herself because if she can't dance with him she's not going to dance with anyone else, because he moves in a way that drives her crazy (not like that, except it so is) and if she can't have him she'll have nothing at all.