From The TimeSplitters Wiki

Shrink is a green orb Power-Up used to shrink the player down to a size slightly smaller than Monkey. The Power-Up is mainly used as an means of escape since a shrunk player is very hard to hit. Due to a glitch, AI tend to shoot over the player instead of at him/her. This Power-Up has no effect in Shrink mode.

Trivia

• There is only 1 Shrink powerup in the whole of TSFP on Vietnam.