Is there a psychological difference between when someone leans back vs leaning forward laughing at something really funny?
A girl i like generally only leans back when she laughs hard at my jokes, and has never leaned forward, but i have seen her lean forward when she laughed at some other peoples jokes.
She may not be completely comfortable with you yet
She could be feeling shy
She could like you but not want to seem too forward
Thus she leans back when laughing