Most high school hitters slow down their swings when they try to hit the ball to the opposite field, usually on a hit and run play or on an outside pitch with no play called. It happens because they were taught the wrong technique such as stepping with the front foot toward the delivery. Not rotating your body until contact is made and exploding your back hip toward the pitch is one correct method. If hitters rotate early they will pull off the ball with their front side and be unable to center the ball on the barrel of the bat and make consistent hard contact.
On an inside pitch we teach hitters to stay inside the ball and lead with the hands rather than the barrel. It is a proven and easier method to hit that pitch to the opposite field.