Leg Drive while benching

Can someone explain how to properly use leg drive while benching? I've tried it with feet out and feet back toward the bench. It just seems like I'm missing something??? How can you tell your using leg drive? Also, it seems like the more I try to use my legs, the more I feel it in my back.
Thanks

First off, my reply may not be what you are looking for, but I will give some advice. I am not a powerlifter, therefore, when doing exercises, my main objective is to develop/strengthen the musculature that I am training. For the bench, I am trying to better my chest. When you use a cheating movement on any exercise it transfers some stress to a different bodypart. By doing this, you are not maximally stimulating the muscle fibers that you are intending to train. I would not recommend to use leg drive or elevating the hips during benching. I have textbook technique, so this is my preference.