One of the members answers:
it seems to me that if you drive with your legs, your butt comes off the bench.
Other posters recommend either moving the legs out wider or moving the legs more under the hips if the butt still leaves the bench.
the idea is to position yourself so you can drive with your legs as much as possible without your butt coming off the bench. Check out the Metal Miltia bench set up for instance. Your feet are curled up under you on your toes, so you can push with your legs without worrying about your ass coming off the bench. There are LOTS of different ways to set up that can give you more leg drive. And it does make a big difference.