The idea that tacit knowledge or information may be stored by an organism (usually human) at a level which is not available to deliberative thought, but which may be accessed better by utilising the physical and movement subsystems - the 'muscles' - instead. A sort of folk psychological explanation for phenomena such as not being able to remember your password until you are sat at the keyboard typing it in. Commonly thought to be induced in the 'muscles' by sheer physical repetition.

Here's a more elaborate example, from real life: I was playing guitar, and just reached the end of a tune, when I was distracted. I noticed I'd played a couple of notes after the end of the tune. I remembered that I'd often previously played another tune after this one, but realised I had totally forgotten the other tune - I had no idea how to play it, and I couldn't even remember what the melody sounded like. Over the next two hours, through a tedious but strangely amusing process of repeatedly playing over what I could remember, starting with those two notes, and many (mostly useless) attempts at self-distraction, I reconstructed the entire tune.

Indeed, learning a tune is for me a process of repeatedly playing it until I don't have to think at all about what note comes next. I (slowly) decrypt, rather than read sheet music or tablature, and so those aren't useful memory aids for me when I want to play a piece. I just have to play it over and over until it's safely stored in my muscle memory.

Other examples: touch typing without being able to name the positions of keys on a keyboard; knowing how to say 'd', 't' and 'th' without being able to say how the movement of your toungue differs between them. In these cases, you can imagine arriving at the answer by mentally rehearsing the movements involved, though I'm pretty sure that wouldn't have helped me with my tune.

Now none of this is to say, of course, that the information about the password isn't stored with our other information, in the brain. What's critical is that to access the information we need to activate those parts of the brain which deal with moving us around, and in some cases this means we actually have to do the moving around in order to get the information out.