Why does a geckos tail still move after its been cut off?

From a functional standpoint, the tail twitches to make a predator think it is still 'alive', or a living creature in of itself and cause a distraction so that the gecko itself can escape.

From a biological standpoint, the nerves and muscles in the tail don't just suddenly cease to function, rather, now separated from the central nervous system, the nerves in the tail fire off randomly. This causes the muscles to spasm, and the tail to move.