In the past I've written about the importance of keeping the hands tied to the back shoulder (more or less) for the major part of the swing. Doing so allows hitters to get the proper bat angle, which is around 30 to 45 degrees rather than flat, at contact.
There's another good reason when you think about it, though. When the hands stay up, it's easier to get the big muscles in the chest, shoulders and core involved for more power.
Try it. First feel what it's like to swing with the hands held high. Then try it by staying in the same position but letting the hands drop to your waist. Do it slowly so you can really feel what's going on.
With the hands around shoulder high you should feel yourself able to drive the bat fairly easily. You should feel strong and powerful.
With the hands more around waist-high, however, you should feel the disconnection. It will likely feel a lot weaker and tougher to bring the bat through. It likely will feel like a lot more work. The bat will likely also be pretty flat as it comes through the hitting zone.
Which begs the question: why would any hitter want to drop her hands to her waist to swing, even on a waist-high pitch? Yet they do all the time.
To really drive the ball, the hands need to remain connected to the back shoulder. Which means if the pitch is low, you have to go down and get it.
Rather than dropping the hands down, bend at the waist. Stick your nose down there toward the ball and use the same basic hitting mechanics you do on a higher pitch.
It's similar to making a throw from a low position rather than standing up. You use the same mechanics, you're just tilted over.
For hitters, bend at the waist, tilt in toward the ball and hit the heck out of it.