When done properly, the bar moves in a perfectly vertical plane only, which is the most efficient path. To do this, one must move the head back to make room for the bar, then replace the head to the normal position after the bar passes by. The lift is complete when the arms are locked out overhead, with the head "through the window", and the feet are reset into the jump stance (directly under the hips). Finally, some other key points to address is the importance of having active shoulders, and a small dip to facilitate the drive of the load upwards. The drop under the bar makes it so the load doesn't have to travel as far to make the catch easier. Note that the dip is not so low as to be a front squat, which takes away the stretch that helps drive the force up enough to pull underneath it during the catch. It is also common to "jump" the legs from hip width into a squat stance, where the hips are about shoulder width apart.
The Push Jerk is one of the nine foundational movements of the CrossFit Methodology.