CherryWoodFuton
Member
When you set up for bench, you both pinch your shoulder blades together and tuck your legs pretty far back. When no weight is on the bar, this gives the effect of pushing you horizontally up the bench. However, the weight on the bar acts as a counterbalance, and the result is it pushes your back up, creating the arch that you see 99.9% of power lifters have. When you have your legs just 90 degrees, you aren't getting any extra power for your lift from your lower body. However, when you tuck them back and push off with your heels, you're getting the added bonus of using your legs and some glutes to add explosiveness to the lift.
This picture gives a pretty good idea of what I'm talking about, although personally I keep my foot flat as opposed to up on my toes and push through my heels.
Yea and heels up is illegal in some powerlifting meets (like IPF)