Leg drive is hard to figure out... when you do figure it out, you'll figure it out on your own, and you'll know it instantly. I'll try to describe it, though, to help you out.
First, lie down on a bench, and without a bar or anything, just drive with your legs. You'll actually move your body down the bench toward your head. That is the same drive you want while benching, just without moving your body. So... it's not going to do you any good if you're sliding around the bench while you're trying to max out, so you need something to catch you, hold you in place while you're using your leg drive. To do this, pinch your shoulder blades together so your entire back is on the bench when you get under the bar. Try lying down on the bench again, this time pinching your shoulder blades together, and driving backward -- if you mess around, you should find a position where you can get your back to 'catch' or 'grip' the bench and keep you from sliding backward, although you may still slide if you use your full leg drive. Now you're on the right track. Now add a third factor to the equation. You get into that position where you have your back as a foundation, and then you unrack a maximal weight. Now your back has 300 lbs pressing it into the bench -- it's pinned down to the bench, and you've set yourself up so that your back is planted there, and it's gripping the bench. Now since your back is pinned down, you can push with full force with your legs and you won't slide an inch.
How does all this help? Imagine if you had a max bench press and you lowered it to your chest. If you could push your body away from the bar rather than pressing the bar up, it would obviously help the lift. This is the same sensation a leg drive gives. You feel not only that you're pressing the bar up, but also that you driving yourself away from the bar with your legs. Another way to describe the feeling is that since you're driving yourself backward and downward with your legs, the bar will seem to go forward and upward -- naturally the opposite of the direction you're driving your body. The bar will feel as if you're actually pressing from a decline... like you're pressing it down and away from you with better leverage.
It's very hard to describe, but if you get on your bench and feel around for it, hopefully you can identify with some of that description and figure it out yourself.
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