RH - I believe the program was 8 weeks total. Good instructor. I believe son gained some positives from it, but also had some issues possibly from it as well as described.
As for the velo increase at that time, I tend to believe that most any focused pitching program aimed at adding velo is going to yield some measurable results. I've read this repeatedly here and elsewhere -- focusing on velo for a prolonged period is going to tend to increase velo somewhat. I think incorporating the concepts into what the pitcher ultimately takes to the mound is more important, and that takes time. BTW, I wasn't suggesting that son grew strictly within the window of the program such that this impacted velo. My point was that year to year, point A to point B, it's difficult to know what is really driving velo gains.
This time a year ago, I was asking the board for input on off season pitching programs... So thought I would share what son did in his Soph off season and our experience with it... FWIW to those at that juncture now.
Ah. OK. I thought you were talking about the measureable distance from baseline to the end of the program. Here is what I personally think such programs provide. The heavier balls may tend to strengthen the smaller muscles in the elbow that support the UCL. Not a lot, but enough to help and certainly enough that the pitcher feels it a little. I remember my son taking the 6oz. ball for the first time and wondering aloud how the one tiny ounce could help anything, but five throws in and he could feel a difference. Where I think the rela velocity increases come from, however, is the work with the underweight ball. I believe it teaches the body to move the shoulder and arm quicker and that muscle memory holds longer than the duration of the program. All of this is just my idea of what may be going on. Just speculation. If I had the time, I'd love to put a group through underweight-only training and see if the gains were, on average, about the same as the group using overweight training.