A horse can be behind the leg without being behind the bit, though.
Behind the leg means that when you put the leg on, the horse's energy does not move forward, but almost sucks backward, such that though it moves forward, there is a feeling of "dragging" and resistance. Even more resistance, the more leg you put on .
A horse that is in front of the leg will kind of "surge" forward when you put a light leg on. It feels like the horse moves briskly forward and in the best of cases, it will "lift" in front the way a speedboat lifts when it accelerates suddenly. Ahorse behind the leg will move more but it's as if it is accelerating with the hand brake on, kind of diving heavily downward and making your work harder and harder fore whatever forward movement you get, and you may even feel "resentment" eminating from the horse's mind.