Well, I don’t think is that hard to explain, LV. When you are making gains, your penis is adapting to a stressor in a given way - growing. If you surpass the ability of the body (specifically, the penis), to adapt in a given way, your bodypart will try adapt differently: supposedly, becoming stronger; if this kind of adaptation can’t happens because work is, again, too high in intensity (or time), than you can have lose of gains and/or injuries.
When you are growing, you have the kind of adaptation you want; now, why do you want to expect to hit a plateau before deconditioning? When you become conscious of a plateau, a different kind of adaptation has gone for some weeks or even months, so we can refer to what ShyMpls said. On the adverse side, if gains are going on, your body is adapting the way you want; what happens if rest for a week?
You can lose a little of what you gained, but this will also be regained easily, because your penis is still in a ‘fresh state’, if it’s clear what I mean; if some of the work was too much when you was growing, it can also happens that in the week of rest you grow.
So, resuming, the logic is that if you rest before hitting a plateau you lose little, where if you expect to hit a plateau for resting, you lose a lot. Just that simple :)
About bodybuilders, I have to say that they don’t plan so rigorously their training for a very simple reason: drugs. There isn’t anymore any scientific way of training in bodybuilding, because everytime a stalling is encountered they just go augmenting doses or using another kind of drug.
But if one trains naturally, taking a week off after some weeks is the way to go. The basic model of weigthlifting training of Russians before the diffusion of steroids, for example, the Verhochansky pyramidal training, was a period of 6 weeks training, 2 weeks of total rest, and another two weeks of very light training to recall previous adaptation.
Old weigth trainers, and also old strongmen, always agreed on this principle: never touch your max in training, or you’ll plateau for a long time.