From The TimeSplitters Wiki

The shrink powerup is one of four powerups in TimeSplitters 2 and TimeSplitters: Future Perfect (information about other games helpful). It is green in color and containes a split-up sphere (possibly condensing molecule). The powerup allows the player to shrink to about the height of a headless monkey. The powerup is mainly used as an escaping tool, although it is known to help in combat, as AI tend to shoot over the player instead of at him/her.