Don’t get hung up on the concept of fatigue, you’ve probably already reached it and didn’t realize it.
Bib used fatigue with a few different meanings, one of which is just the feeling of target tissue damage. Anyone who has gained has done target tissue damage whether they consciously felt it or not. So they reached fatigue. Which is the next use of the word. Reaching fatigue means doing PE beyond the elastic range. And Bib often used it to mean the ultimate stress or even the fracture point on a stress-strain curve. So it refers to a range of points on the stress-strain curve, leading to confusion. Then he also uses it to mean an accumulation of damage, because we’re dealing with multiple levels of collagen structures. A single high intensity workout can cause collagen fracture and failure in a short time frame. A series of frequent low intensity workouts over days or weeks (or months in Bigger’s case) that barely cross into the elastic range, can stretch fibers and structures out so far that it takes hardly any additional marginal stress for fibers to fail. This state of fibers failing at relatively low stress levels due to time, frequency, and temperature is also fatigue.
Rest days allow target tissues to contract back to their normal size - since they are living viscoelastic structures, this takes some time. Just like it takes time and frequency to get to the state of fatigue, it also takes time for the tissues to revert. When Bib talks about tissues healing back stronger because of a rest day, he is mostly referring to this contraction. However there is also an actual healing effect going on too. Somewhere around 36-48 hours since the last workout, proliferation overtakes inflammation as the dominant process.
Bib’s idea and theories for methodically doing controlled damage to the target tissues are brilliant. But his advice to take literally zero rest days has to be viewed within that context of causing damage. Of course a rest day is going to interfere with causing damage - it drastically lowers the frequency of the stretch. And from that state of fatigue accumulated over days or weeks or even months in his case, of course a rest day is going to give the tissues a break, and after that rest they may appear to be “stronger” in the sense that more weight/force is necessary. But it’s only stronger relative to that extreme state of fatigue that Bib is referring to.
Rest is necessary for the vast majority of PE’ers. Tissue growth happens during rest. Check out the IPR model of wound healing. Rest and deconditioning actually lowers the force levels necessary for gains, as opposed to making the tissues stronger. I’ve seen a lot of success using Bib’s damage methods for several weeks at a time. But then I take a decon break. With my hanging, every cycle like this I’ve done has been between about 2 lbs and 8 lbs. I start at 2, about a month later I’m up to 8. I get some gains, take the rest of the year off from hanging. The next year I start again at 2 pounds, and gain again on the way back up to 8. Healing back stronger from rest days is an illusion.
Continually damaging the target tissues for months on end only works for some guys - and they’ve gotten good at keeping the rate of damage very low. For most guys, it is actually one of the only ways they can truly injure themselves, because over that time frame and frequency, it becomes possible to damage macro collagen structures. They keep a high rate of damage for a long time. Chronic inflammation, or non-healing wounds, is no good for any part of the body. It’s why I take regular decon breaks.
The allure of Bib’s methods is strong - gaining 5x faster than most guys would be nice. Maybe it’s possible, I never found it, still looking sometimes, but I hope you do. Just remember that your body needs to be able to heal and repair that damage you do each day. And if you do too much damage that the repair systems can’t keep up, then it’s time to take a decon break. Balancing the damage and healing is how I think those big gainers who took zero rest days did it.