From The TimeSplitters Wiki

Doctors say that injection won't hurt a bit, will try saying that to the guy next to me.

Injector

The Injector is a weapon that can elminate your enemy with 3 shoots. After 3 shoots, it causes your enemy to explode. This weapon is almost like a croosbow, you can always pick up your ammo from the wall if you miss the enemy. But remember, you must hit your enemy with 3, anything less will just hurt them and make their screen become blurry for a few seconds. This weapon only appears in TimeSplitters Future Perfect.

Elite's Tips

Only use this weapon as a backup weapon and an ownage weapon when you use it right. When you have it, don't hold the fire button, but rapidly press it so you will have a better chance of making your enemy explode,

For more Tips, Visit the Weapon Tips page