As a drill used once in a great while, to have players get the feel of driving throught, no problem. But as a daily or weekly drill with lots of reps is NOT a good idea for players 14U and younger.
It puts too much stress on the wrist and shoulders of children. I wish I could find the link to the study that talks about this.
One day, put it away.
If my high school coach had his girls do this drill, some would have a sore something, because you must teach at the bag, you cannot just HIT IT.
This drill, I use to Teach. how to SIT INTO YOUR COIL, as you pull back into ROCK THE U. How to stay connected. how to TILT.
GRIP (don't start) contact positions. A GOOD PIVOt, HINGE, SERVE THE PIZZA.
Do not hit INFLATED BASKET BALLs, Volly balls, Footballs, EVER! Dead weight, with supervision.