Lower the bar down to the knees, as if you were doing a Romanian deadlift.
Explode into a vertical jump while holding the bar. Do not pull the bar. The arms stay straight.
Don't lower the bar too slowly. Just keep it under control. Be sure to finish the hip extension; make yourself "tall." Don't be surprised if you jump backward a bit when you get it right.
I'm going to describe a little 6-week program that I call Tsunami Training (I'll get to why I call it that a little later). It's based, for the most part, on training methodologies that I picked up from Ian King and Charles Poliquin, with some razzle-dazzle thrown in by myself.