The answer to the question depends on what happens in the swing prior to release -- i.e., when the bat contacts the ball.
There are only two things that determine how a ball is hit -- the track the bat takes through the air, and the speed the bat is traveling upon impact. What happens after impact is only meaningful if it affects the track of the bat or the speed of the bat at or before impact.
Some good hitters hold the bat with both hands all the way through he follow-through. These tend to be players with good flexibility. Their arms and their spines can flex as the bat decelerates.
Other good hitters release their top (or back) hand after full extension. These may be more muscular, less flexible players.
My point is, whether the bat decelerates in one hand or two generally has no effect on the swing up to the point the ball is hit. The player will usually arrive naturally at the deceleration technique that works best for her.
There are good hitters who do it both ways. It's not something I'd spend much time on unless a player's release is so awkward that it's slowing her bat down, or unless it looks like she might injure herself.
I would like to offer another view point. At the point of contact if we are not hitting the ball square it is wasted effort in my opinion as we are not driving the ball and the bat makes a thud sound verses a sharp click or crack. This usually is a result of grip, as most teach lining up the finger knuckles and holding the bat in the back of the top hand, so at MOI or when the wrist unhinge the bottom hand is not being used efficiently and the top hand wrist is probably rolling over and we feel this is important as the ball is only on the bat for about 2 milliseconds.
Most girls have no idea what that feels like to hit the ball squarely or for that fact what it feels like to pull on the bat and unhinge the wrist to make square contact which is why we take the time to show them what leverage is and how we grip the bat with both hands makes a difference. Then we show them how to use a hammer vertically and horizontally with each hand independently and then with what we call a hammer bat. The mallet head is attached to a shaft like a bat and the mallet head is about 27 inches from the knob or the sweet part of the bat.
SBfamily tried the TCB weighted balls and this is yet another way to demonstrate why two hands are important. Most hitters can not get the ball past the pitchers rubber and most of our kids can get it almost to the grass. The ball does not deflect and the trampoline effect is lost, so it is all in the hitter now. The Static families bought me 6 of these balls and so far I like the results. However I would not use anything but the 14.99 ounce ones.
Most women golfers have problems with their swing and follow through because of the carrying angle of the elbows which can be as much as 4 to 10 degrees. Their shoulders are narrower and rounded and are looser compared to a male. Simply have a female hold their arms straight out in front of them and palm up and you will see the differences. The thumb side of the hand angles outward and this is why they hit like a girls and another reason they throw like a girl because they are a girl! However it can be taught and this is why when we do the hand over drill they usually hit the ball harder with just the lead hand because it is new to them and they feel it probably for the first time. Then when they use both hands correctly they hear the bat click and we hit the ball harder and farther. We teach to hit the ball squarely with both hands and drive to the ball and through the ball and the rest normally takes care of itself.
In most of the younger ones they do not turn and tilt enough so they release the top hand first as you can see visually the baby finger and ring finger and social finger begin to come off the bat. This is why keeping both hands on the bat is not easy for most girls as we as males never taught them how to use an axe or even how to use a hammer and that means little to no use of the wrist and grip is not even taught.
Ask your daughter to very slowly swing at a ball on a tee and not hit the ball and when the back elbow is coming down pay particular attention to the top hand fingers. I mention this because if the shoulders are not tilting and turning you will see them come loose from the bat as they are too linear.
I did this yesterday with three college coaches who have played ball and they were pulling everything and we changed the grip and did as described above and you could see the difference in bat speed and hear the difference as they were hitting the ball squarely.
I have girls who can not afford the $300 dollar bats that use the Michelle Smith Rip It bats that hit the ball 240 plus feet. It is not the bat in my opinion it is how they are taught to use it that makes the difference.
