"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.
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.
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.