I figured out how to utilize this "leg drive" during bench press sometime during the past couple of months, and I'm sure it has contributed to my continued gains.
I discovered this by using my *imagination*:
Maybe this is totally wrong, but it seems to be working for me so far.
Here's what I do:
What I try to imagine is if there was a bathroom scale placed underneath my upper back, buttocks and one for each foot as I am lying down on the bench.
I then push with both of my legs and raise my hips so that my butt is OFF the bench. Then I lower but buttocks gently so that they are barely touching the bench.
If I pretend to have an out of body experience and take a look at those imaginary bathroom scales while I'm pushing into the ground with my legs, I would see that most of the weight distributed on my feet and my upper back, with nearly 0 lb of pressure being placed by my buttocks.
It's almost as if I'm trying to lift my butt off the bench, but just not quite there yet.
In this position, my quadriceps, glutes and back will be contracting hard and my back is arched and my butt will be touching the bench ever so slightly.
Even before unracking the bar, my is body is tight and uncomfortable.
Combined with the rubber anti slip mat on the bench to prevent my body from sliding, imagining "bathroom scales" to harness the power of leg drive has helped improved my bench press.
Works for me, and it could work for you!