If you have a good arch and your glutes are engaged it's gonna be much harder for your butt to leave that bench... Also while you're pushing your knees out, you want to be engaging your quads. As soon as I'm rack the bar, I'm probably at about 20-30% legs engaged. As I lower the bar, I go to 40%, 50%, 60%, 70%, all the way up. As soon as the bar touches my chest and I reverse it, my legs are 100% engaged trying to slide me backwards on the bench. But since my traps are set and I'm pinned, my arch is going to hold and all that kinetic energy is going to go from my feet through my legs, into my torso, up my arms, and out the middle of my hand just like a punch.