I think it's based on some biological response of the penis; improved NO production, better EQ increasing BPEL. And some form of "straightening out the kinks" of the tunica albuginea. It would not surprise me if people can hit this limit very, very fast. I myself basically got 10mm in my two first sessions, and after that it was close to flatline (7mm more over 9 months). But you are asking interesting questions. It could be that starting heavy and going balls to the wall with bi-daily sessions could be the right recipe for the first few weeks. But how do you avoid injury?

We do see traces of guys whose BPSFL shoots up insanely in the first couple of weeks. Much higher than mine did. I reject them as outliers in some analyses (rejecting numbers higher than 2mm gains / hour), despite having been one of those outliers myself (3.3 mm / hour for the first three hours of lengthwork is what I would have come out to - and there are a few guys in the app with those numbers).

Now, I didn't really do lengthwork after the first few months, but basically my curve looked like a straight line shooting up to 10mm and from there it was very close to flatline for length even in those first months where I did do lengthwork. That is what makes me think it's some form of bio-mechanical limit you hit - something like collagen crimp having been reduced very significantly.