There are very few hitting faults that can be corrected during the game. The job of a hitting coach isn’t necessarily to teach during the game, but to motivate and encourage his players. Practice is the time to teach. However, there are subtle points a coach can make that can help a struggling hitter.
As a hitter, you want to be able to control the things that you can control. First is your stance. Second is your approach or stride toward the pitch. Third is your follow through. A breakdown in any one of these areas can cause a hitter to take poor swings and lose confidence. The only thing a hitter does not control is where the pitch is thrown.
So as a coach, I would choose one controllable aspect and plant a seed in my hitters’ mind, in between at bats, and explain why. Some examples of this may be to move his hands to a different spot, make sure stride foot goes back toward the pitcher more or make sure you are getting a full follow through. Blindly yelling instruction from the dugout or from the third base coaching box during an at bat will only divert the hitters’ attention and cause him to lose focus.
Sometimes, a slight change in philosophy may help as well. Start stride a little later if too far in front of pitch and hit the ball the opposite way, start stride a little earlier if too far behind pitch and try and pull the ball, try and hit the ball back up the middle, etc. Try not to give the hitter more than one thing to think about during his next at bat and make it something that he can control.