To elaborate on Krollmonster's point about the importance of a solid rack:
The whole idea with the jerk is that you aren't pressing the bar over your head so much as you are pushing yourself down very quickly underneath the bar. The bar will rise some during the jerk, but not much.
You dip down, then propel the bar upward, and ideally you lock your arms out under the bar at the exact moment that it has lost its upward momentum, when it is momentarily weightless, right before it would start coming back down. At that same instant, your feet hit the floor in the split position.
Now, about that rack. To achieve the upward momentum that will propel the bar, you must use your legs. If you are holding the bar in a military press position, then when you drive upward with your legs your arms and shoulders will absorb much of the power of that leg drive, so it will not transfer to the bar. On the other hand, if the bar is racked across your collar bone, there will be a more direct transfer of power from the legs to the bar.