If you push your head towards the direction of the bar, there's no way that your butt should pop up off the bench. When you take the the bar out, tighten your quads and bring it down to your chest... Take [the bar] down with tight quads, but then when you're ready to throw it up - that's when you're going to really fire your legs to push yourself back towards the bar. That's going to help... raise your chest towards the bar and... apply all the force of your legs towards the bar.