[FSCJedi]

Here's a good explanation I came across years ago:

Tachypsychia (literally: the speed of the mind): the distortion of perceived time. In a life-or-death situation, the mind kicks into overdrive and transfers into the sub-conscious (which operates much faster than the conscious) so your perception of time becomes altered. This causes the observation that things are happening in slow motion (a.k.a "Bullet Time"), even though you (and your opponent) are probably moving faster than you ever have. Tachypsychia can also work in reverse ("It all happened so fast"). Observations have been made that the more experienced and highly trained a person is, the more likely that person is to experience tachypsychia. That is, a person who knows that "trouble happens" is less likely to be surprised by it, and more likely to respond with super-heightened awareness.