From The TimeSplitters Wiki

The injector is a gun found in Timeplitters Future Perfect. It is a small gun that fire tranquilizer darts. If three or more hit an enemy quickly enough, the will inflate and explode, and the explosion hurting anyone too close. The Tranquilizer darts can be picked up if they get stuck in a wall or floor. Any player hit by any amount will begin to inflate, but it will stop if not hit enough times. While inflating from one or two hits, other darts can be put in to cause them to explode. The injector does not follow auto-aim.

Noise Glitch

The inflation sound from when a player is inflating can get stuck. It usually only happens if an inflating player is killed, but it rarely happens. The infaltion sound gets stuck in a spot, so the closer you are to it, the louder it is.

Role in TSFP

In the Story level You Genius, U-Genix, the injector is the only way to kill The Freaks. It is also featured in the Arcade level Chinese's recommended weaponset.