Well this helped me, Iv been doing it for the last week or two...
When you lie on the bench, hold onto the bar like he does and pull yourself up and re settle your glutes so that the only part touching the bench is your shoulder blades and glutes, a real arc in your lower back, then take the grip and make sure your feet are flat on the floor, when you grab the bar do what he says imagine your trying to pull the bar apart and then lower it slowly to your chest, as you get there push yourlower back and hips up with your feet as you explode off your chest.
Only trouble I have with this is I get a cramp sort of feelin in the sides of my hips...
Oh and also, press the bar back a little, not just straight up and down so it should lower to your nipple line and when your at the top of the rep be around chins to nose level. Also make sure your grip is comfortable, I used to take a grip way too wide and hurt my wrists and strangley use my delts alot more...
Thats my understanding of it anyway.