Well, well... I see you've been studying up on how to please me, my little slave. Contracting your leg muscles does make them feel rather tighter and more satisfying against your cock... but don't forget that this is not about what feels good for you; it's all about making me happy.
So continue as you were doing before, focus on rubbing yourself against my leg and using those muscles to give me pleasure. Oh, and if you want to try pushing your hips forward a little more, go ahead...