Sandman,
Mankin doesnt have to swing a bat into a 300 bag of sand - with the musculature of a young boy.
In fact - Jack aint swinging anything into a heavy bag. He's just talking about it and letting other folks give it a poke.
Its sad to read what he and others write on this subject - and if you really think about the timing required to do this heavy bag drill without injury - you may change your mind about its appropriateness for a growing young boy.
If the timing is wrong - (which i would guess in 99% of the cases it would be) you have a major problem.
Its just sad for me to read this stuff some times.
Sorry for the opinion against the stuff. I am not trying to be offensive - and I sincerely hope it works for you.