Vizualization is a technique in which you imagine yourself performing. It is often done from a 'first person' perspective (from inside your body), but can also be done as if you are watching yourself from outside your body.

It works because the brain 'doesn't know' the difference of you actually 'doing a movement' or just imagining yourself 'doing a movement.'  Your muscles actually 'fire' like they do during action, but at a teeny, tiny, miniscule level.