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Every player in the world, whether playing at an elite or beginner level, have very different golf swings but they all strive for one thing; the perfect impact position in order to send the ball as far as possible.
Elite golfers in particular share this trait whereby their golf swings may differ but their impacts are very similar.
Ideally, the set up position should have the head behind the golf ball with the body weight more central. During the swing, the head will shift a little to the right with the weight transfer and then drive back towards its original position.
If a player squats a little from the top of the back swing to produce more power, then the head will drop three to four inches in height. Height will be regained through impact as the hips turn and the left leg straightens, however, the head will not go past the ball at any stage.
To check and measure this, try using swing analysis software, and be sure that the left ear is in line with the golf ball at address and then again at impact. If the head starts or goes past the golf ball during the swing it will have an adverse effect on the impact position and therefore the outcome of the shot.
In the driving range, practise hitting balls with your head positioned behind the ball but be sure not to keep your weight on the back foot. The weight can still transfer through with the head back because the hips, hands and arms can lead in a powerful position and create a stronger trajectory.
This is the case with all clubs on full shots. The only time the head gets ahead of the ball is when chipping or playing specialist shots. The head will be further behind the ball at address and impact with a driver because of the width of the stance and ball position.
Some players choose to add more weight to their back foot which would bring the head further behind the ball, tilt the shoulders upwards for a high ball flight, but again the head will certainly not get ahead of the ball.
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If the head gets ahead of the ball at impact, it will mean the hands and arms have to counteract this movement. If they manage to do so, the ball flight can vary between a higher or lower than expected shot due to the swing path but the inconsistency with the ball striking will certainly be the dominant factor in this swing flaw.
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The head movement before, during and after the golf swing should be consistent, if the head is behind the ball at impact with one club; it should be there with all of them. In this instance, your head should not be ahead of the ball with the longer irons due to the fear of increasing the angle of attack and catching the ball heavy or potentially skying the driver.
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The head may lower in height during the swing, particularly if a player squats into the ball to start the transition from back swing to down swing, however, the relationship between your head height at set up and impact should be nearly identical, or certainly you should have that intent.