Forward hip movement in the initiation of the press has multiple purposes. First, it helps move the head out of the way of the bar as it goes up, so that we may press in a more or less straight line from start to lockout. Second, when properly timed, it initiates an elastic rebound that propels the weight off the shoulders, much like the stretch reflex at the bottom of the squat. Lastly, it allows the use of more musculature by better aligning the front deltoids and pectoral muscles with the direction of bar motion. In order to reap these benefits, we must execute the movement correctly.
Correct hip-forward movement at the start of the press shifts the weight slightly to the toes as the torso inclines (left). Leaning back without moving the hips forward (right) sets up a rebound that will move the bar forward instead of up.
A common problem with the press is confusing hips-forward with leaning back. Both of them will get the head of of the way and initiate a rebound, but leaning back will make the bar rebound forward of the shoulder, rather than up and backward as desired. With a heavy enough weight this is not recoverable, and leads to missing a rep that would have gone up with better technique.
The easiest way to tell if you are moving the hips forward or leaning back is to pay attention to where your weight is distributed on your foot. Try this simple experiment now. Stand up facing a wall a couple of feet away, put your hands on your hips, squeeze your quads, glutes and abs tight. Now look up at the ceiling and lean back away from the wall. You will feel your weight shift back onto your heels. Next, go back to the starting position, keep your gaze forward and reach toward the wall with your hips while staying tight. If you do this right, you will feel your weight shift toward your toes. This is the way you should feel when you drive your hips forward at the start of the press. Try it with a bar in your hands next time.
If you can video yourself pressing, you will be able to distinguish leaning versus reaching by watching the bar as you move the hips. If you are leaning back, you will see the bar move back and forth horizontally. If you are reaching forward with your hips, you will see the bar move mostly vertically – downward as the hips reach forward, and upward as the hips return back to their neutral position. It is important that the hips return to this neutral position after every rep, so you can start the next rep by moving them forward. It is tempting to leave the hips forward, because it is a more relaxed position, but this leads to a loss of hip movement as the set goes on.
One last point: it is easy to lose track of elbow position while moving the hips back and forth, especially if the weight in your hands is heavy. Allowing the elbows to drop will cause the bar to bounce forward away from the shoulders instead of upward, much like leaning back would. If you are new to moving the hips while pressing, you may need to exaggerate keeping the elbows forward and up, and keeping the chest pointed up at the ceiling until you get the hang of it. This ensures that deltoids and pecs join together to drive the bar straight up overhead, keeping the bar close to the shoulder joints and adding muscle mass to the movement.