I'll try to expand on it a little more.
You should press the bar up to a certain height. My progression usually starts with the bar roughly three inches above my head, then to the top of my head, then to about eye level, then from resting on my shoulders. Your feet will be positioned like they are during the jerk drive; you can even get on your toes to simulate actual extension.
From those different heights, you're going to push yourself underneath the bar and get into a split position; that is, you're NOT driving the bar up any further. When I see people do this drill, they tend to push the bar upward, which defeats the purpose of the drill. Your goal is to push yourself DOWN. The height the bar starts is supposed to simulate the height the bar traveled up from your jerk drive. The depth of your jerk will depend on the bar's starting height.
Remember that we want to beat the bar down and have it in a position overhead where we're most stable and able to support the weight. We want to receive the bar (the same applies for the snatch and clean) at that moment of weightlessness; any downward force (damn gravity) when you're receiving other than when it's in the proper receiving position (cradled overhead in the snatch and jerk, or on the shoulders in the clean) is going to either a) throw you off balance and make your recovery harder, or b) cause you to miss the lift.
I hope that helps you understand the drill a little better.
Javier A. Sanjuan
Olympus Barbell Club
Dear God, please help me lift heavy and be awesome. Thanks. Amen.