I find as I get more practice I use the hips in different ways. When I first started I didn't get much power out of them, as I go on I get more and more, because I use them in subtly different ways. I want to know how to get the most out of them? Also how do you know when you are using them optimally?
Its in your DNA. Some people have strong and mobile hips naturally. Punchers are born. However you can practise to streghtening your hips through exercises and stretching. To be able to hit hard you need explosive power and speed to your hips. That you get by doing squats, fast sprints, jumping. I'd say wrestling is also very good way to strenghten your hips. Use your imagination. To get your legs behind your punches try more sitting when throwing punch. Sit when you throw punches, push your legs down like squatting. But keep the movement forward.