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A strong left hand grip is extremely important as it affects directional control of your golf shots.
A strong left hand grip in golf means that you have not positioned your left hand on to the handle of the golf club correctly. A strong position with your left hand means that you are holding the handle of your golf club with your left hand rotated too far round it in a clockwise direction.
Your hand position on the handle of the golf club, and therefore, how you grip the golf club is absolutely crucial for controlling shot direction, ball flight and therefore your accuracy when you are out on the golf course. Your hands are the only connection that you have with the golf club and it is the golf club that connects with the golf ball, so being able to control how the club face strikes and connects with the golf ball means that you are able to control the flight that you then see the ball take.
If you begin your swing from a position where the club face is aiming at the target and you are able to return the club head back to the ball through impact with the club face still aiming at the target, then you have a great chance of hitting golf shots at the target, provided the movement of the club head is also down the target line towards the target.
Being able to present the club face back to the ball at impact, so the face is aiming at the target, is directly influenced by how you hold the golf club. Your hands have a natural position that they want to be in. If you stand with your arms and hands relaxed by your body, you will notice that your arms hang slightly in front of you with your hands in a palm inwards position to your body. This is your hands natural position that they want to be in. How you hold the golf club needs to replicate this natural palm inward position as closely as possible. If you have a strong left hand grip, you are holding the golf club with your left hand rotated clockwise beyond this natural position.
As you swing the golf club and move your hand through impact, your left hand will rotate back to its natural position and the club face will rotate more to the left than it started and closing the club face in this way will result in your golf shot missing the target to the left.
A great way to check if your left hand is on the handle correctly is to take a coin and place it between your left thumb and hand. The coin should be on the front of the handle and in a position where it is pointing up towards your right shoulder. This means your hand is in the correct position and you have a neutral left hand grip.
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If your left hand has rotated too far around to the left on the handle of the golf club, it is actually in a weak position not a strong one. This will give you the same directional control issues, however, the ball will tend to fly right of target from this position rather than left.
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A strong grip has no relevance to how tightly you are holding the golf club handle. Grip pressure is a separate issue to hand position on the handle. A strong grip means your hand is too far around the handle to the right.
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A strong left hand grip will actually produce shorter golf shots rather than help you to hit the golf ball further. The strong grip will result in the club face closing and rotating left of target and you will experience a low ball flight to the left of the target and the ball will land earlier than usual due to the lower trajectory and therefore will not travel as far.