Hitting behind the ball
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Good players have something like 90% of their weight on their forward foot at impact. That's rule number one towards not hitting fat and thin shots. The second rule is understanding where the handle of the club is at impact, which is governed by the angles created in the player's wrists.
Fat and thin shots are the product of the same issue: the swing's lowest point is behind the golf ball.
All good players strike the ball with their weight predominantly forward and the shaft of the club leaning forward (towards the target) at impact.
Example of a player who flips, leaves his weight back, hits it fat and thin, and generally plays awful golf:
Notice the bend in his left wrist. Notice that his hips have barely moved laterally. His weight is predominantly back and not even close to forward enough. The club's shaft is leaning away from the target.
Here's an example of a player who almost never hits it fat:
Weight is a lot more forward, there is significant bend in the right wrist, and the left wrist is flat. The golfer has forward shaft lean. This player probably hits his driver 100+ yards farther than the player in the first picture.
Keep in the mind that the head must stay relatively stable as the weight shifts forward. So the hips essentially slide with a steady head.