From The TimeSplitters Wiki

Shrink (Powerup) is a glowing green sphere that contains what appears to be a segmented orb. It temporarily yet dramatically shrinks the player, reducing their height to less than that of a monkey. With this power-up, enemies find it incredibly hard to hit the player because the user makes for such a small target.

Trivia

• There is only 1 Shrink power-up in the whole of TSFP. It is found in the Vietnam level.