In theory, I think "instinctive training" does hold water. But in practice, for most people it's probably counterproductive. This doesn't mean you can't adjust your training day to day a little bit — to account for feeling better (going for more reps on a final set) or feeling worse (just doing the required workout and leaving). But to rely entirely on your instinct requires two things:
- A person that is smart enough to assess his or her own training and monitor their body.
- A person that is strong enough to bypass most traditional training programs that have been proven to work effectively since the dawn of the barbell.
These two things are rarely found in the same person. The smart ones are usually never dumb enough to push the insane limits of their body. They may be strong in their gyms but never really strong in the eyes of the elite (or even the "sort of strong but probably never really been accused of taking anything"). They always kind of swim in the kiddie pool, safe in the piss warm water of mediocrity.
But the super strong ones are never smart enough to stop the insanity. These guys get shit on by the "smart ones" but usually only behind computer monitors because, well, these guys would probably kill them. (If their bodies can hold up long enough to fight.)
So to recommend instinctive training really isn't the best idea. What you need to look at is your training program. A good training program will allow you to progress with the ebbs and flows of your day and how you feel; a poor training program rarely gives you an "out" — you better be on your best game or expect to be highly disappointed. If you're following such a program, either write better programs or find a new mentor.
But here's a closing rant: one thing that no one talks about is how walking into a weight room tired, over-worked, stressed, and fucked up but then succeeding is a true lesson in mind strength. I don't always recommend it, but at certain times of your life you've got to quit being such a slave to your excuses and step up and see how well you can do under shitty conditions.
People have always been working labor jobs for long hours. There are single mom's working two jobs and still finding time for their kids. There are soldiers that fought for our country that were hungry, tired, thirsty, and with pieces of steel stuck in their bodies, but somehow they were able to pull it together in combat. Why can't you get your shit together for an hour and do a couple sets of squats?