A Malaska Golf Member has a question about grip and your left thumb. Mike says from his experience, your left thumb can be the biggest killer regarding tension.
Mike explains that when you hold a baseball bat, like tennis, your thumb is not on the bat. This makes it easy to load and release the bat. There is no tension in your wrists.
When you hold a golf club, the thumb is slightly on the side of the shaft, so it is a support mechanism when you get to the top of your swing. Also, in the downswing, your thumb prevents the club from swinging all over the place. When the club gets outside your hands on the downswing, your thumb is now along for the ride.
It is important to note that your thumb does not push down into the club. On the follow-through, the club rehinges, and your thumb stabilizes the club again.
Mike, say that some players would really benefit from taking their thumb off the golf club like you would on a baseball bat. Swing with this grip and feel how loose your left wrist feels. Now put your left thumb back on the club and feel that same swing as you did when you had the thumb off.
Again, don't push with the thumb. Many players that Mike teaches grip the club too tightly. Often Mike will take their right hand off the club to look at their left hand. He notices that he can't move their left thumb, and it's almost like they have a "death grip" between their left finger and their left thumb.
When Mike grips the club, there is no tension in his left thumb, and he is not pushing the thumb into the club. It is just sitting there acting as a stabilizer. Many players hit the ball fat because they use their thumbs and push the club down.