From The TimeSplitters Wiki

A glowing green sphere that contains a divided ball. 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, often firing miles over their head. Like all power-ups, it is only available in TS2 and TS3's arcade and mapmaker.

Trivia

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