Rule 4 of 25 · Chapter I — Before You Move
Stand with your sternum over or just ahead of the ball, never behind it.
Why this rule exists
Your chest position is the best single predictor of where the club bottoms out. Lean back at address and the low point moves behind the ball, so the ground gets hit first — the classic fat shot, and the reason many golfers scoop. Setting the sternum forward of the ball keeps the swing's low point where it belongs, ahead of the ball.
In practice
With the club soled behind the ball, glance down and check that your shirt buttons or the centre of your chest sit level with the ball or a fraction target-side of it. A slight tilt of the upper body away from the target is normal for the longer irons, but the chest should still not be behind the ball. If you tend to hang back, feel a little extra pressure in the lead foot at address as a counterweight.
When it doesn't apply
Uphill lies require you to tilt with the slope, which will move your chest back relative to level ground. Tee shots with driver are the opposite arrangement entirely — there you want the chest behind the ball to hit up on it.