I once met a guy who practiced aikido and explained a little about it to me. Aikido is a martial art, but it focuses in particular on what I'd think of as "body hacking." There are ways to make your opponent trip and fall, or use his strength against himself; there are ways to pick up and throw a much heavier and stronger opponent. Of course, because of this, there's often a lot of mystery and hokum surrounding aikido, but this guy said he belonged to a particular branch where, instead of venerating tradition, they try to come up with new "body hacks" and have worldwide meetings to test and compare them.
The relevance to LessWrong is tangiential, except that this seems like a model of rationalism applied to things that seem like magic: it's a good example of how one should behave when faced with the inexplicable.
This dude demonstrated (and taught me to perform) a "trick" that I would not have believed possible.
First, hold out your arm straight and stiff and ask someone to push it down. You can offer resistance, but if your opponent is strong enough, or if enough people are pushing down on your arm, you'll eventually fail.
Now, instead of holding your arm out stiff, make it as loose and floppy as you can. Then hold it out, but think of "reaching" through the air, but without moving. (As a visual aid, Aikido Guy held out a Coke bottle and asked me to hold my arm out while thinking of reaching for the bottle.) Now get someone to push your arm down. You are much, much stronger. And you don't feel like you're expending any effort. Nobody could push my arm down. Aikido Guy, who is very small, told me that he'd once gotten three varsity football players hanging off his arm without budging it.
Aikido Guy was curious about this mystery, and luckily he was on a university campus, so he asked to be wired up to an MRI while he tried holding his arm out both ways. Apparently there was a real difference in the way the neurons fired. When he was trying to hold his arm out "stiffly," his brain stimulated all the muscle fibers in his arm, but intermittently (compared to the timescale of neuronal activity.) When he was "reaching," his brain stimulated a smaller fraction of the arm muscles, but continuously. Apparently the former kind of muscle stimulation is more tiring and weaker than the latter.
So, although we don't really understand what's going on, we do know that there's a physical explanation for it. (Do any of you have a better explanation?)