Batter's shoulder is a problem with a hitter's front or lead shoulder that is caused by finishing the swing one-handed; by letting go with the top hand.

Batter's Shoulder

Followers of the late hitting instructor Charley Lau are told that the key to power is finishing the swing by taking the top hand off of the bat and with one hand on the bat. It is said to be so important and distinctive that a one-handed finish is often referred to as a Lau Finish.

While the value of a one-handed finish is debatable, given that the top hand doesn't let go until after the ball has left the bat, in my (direct) experience, finishing the swing by taking the top hand off of the bat can put the batter in a problematic position and sometimes lead to shoulder problems.

Hanley Ramirez

On occasion, my son will complain of pain in his front shoulder, and the pain always goes away when I tell him to finish his swing by holding on to the bat with two hands.