"Let Go Of The Bat With Top Hand"
With the better hitters, all bat head acceleration occurs before contact. The follow-through is actually the "coasting out" segment of the swing. The traditional swing would have the batter finish with both hands on the bat. But many coaches think it is beneficial to a player's mechanics to let go of the top hand after contact.
Those that teach "throwing weight onto a stiff front leg" will also probably find it necessary to teach letting go of the top hand. For the batter to keep both hands on the bat and have a full follow-through, the hips and shoulders must be allowed to freely rotate. At the completion of the traditional follow-through the shoulders will have rotated too (or past) the "facing the pitcher" position. This allows the lead elbow to release down-and-in and the hand-path to arc back toward the catcher. But, if the batter's hip and shoulder rotation is limited, he will find it necessary to release the top hand.
Keeping weight on the back leg and having the body slightly tilted away from the pitcher will allow for full rotation. Throwing weight onto a stiff front leg and bringing the body to a more vertical position will have the effect of limiting hip rotation. You can check this out for yourself. --- Stride with the front toe closed. Now, cast your weight onto a stiff front leg and note how restricted your hips are.
Many batters let go of the top hand in varying degrees. It is one thing to let go by choice but if your mechanics are forcing you to let go - you may want to make some changes. --- Swinging at an outside pitch can also result in less shoulder and hip rotation and therefor the need to let go of the top hand.