Fatigue is a good indicator you’ve stressed the target tissues enough to cause growth. It’s not a necessity, but I think if you don’t sometimes feel it, you’re not quite toeing the line of the correct force thresholds and work volumes for post-newbie conditioning. That’s just my experience.
When I was hanging, I experienced it mostly in the ligaments and lower abdominal fascia, and to a lesser degree in the tunica.
I’ve also experienced it from almost every other form of PE I’ve done. Any exercise can cause fatigue with enough volume and force. The difference was that when I was hanging, I didn’t take rest days, so I was feeling fatigue constantly.
Bib’s concept of “riding the fatigue” in hanging is essentially aiming to cause fatigue every day. There are a lot of very good old threads discussing the mechanism of PE growth, and the role of rest. I’m in the rest camp. In other words, I think it’s important to seriously stress the tissues, to feel fatigue; then I think it’s important to rest afterward. But extremely high volume does work for some guys, and I did gain from Bib-style hanging. I think macro level rest, i.e. decon breaks, are more important than micro level rest, i.e. off days, in terms of gains. And that’s why Bib style hanging worked for me. I think micro level rest is more important in terms of general penis health.
Even Bib theorized that the growth in PE happened during rest. He studied the anatomy of collagenous tissues, I’ve looked at some of the info superficially. He theorized that while he was hanging, he was pulling collagenous fibers into a straight aligned position, and while he slept they formed new crinellations, or tiny folds of new growth in the fibers. And then the next day he would pull them straight again. It’s a simplified explanation of a complex biological process. So he acknowledged the actual growth happened during rest, he just believed that 10 hours was a sufficient and ideal rest period.