A hitter's power comes from the bottom or pull hand. This drill develops bottom hand strength. Have batter, using only one arm at a time, take stance, inward turn, stride and full cut at balls off of a tee.
Emphasize "staying back" on the ball. Batter will need to choke up a lot, and initially need to place the unused hand under the armpit of the swinging arm for additional support.
Take about 20 cuts at a time with each arm, making sure to keep the barrel higher than the hands, and follow through. At first, most players will find their top arm to be stronger; therefore, requiring more reps with the bottom arm.
The objective is to reduce the gap between the ability of each arm, ideally becoming equally adept with both. You can soft-toss to older players.