Grenade slide

This is a glitch in Halo: Combat Evolved.

Tutorial
To do this, you need a partner, a grenade, and room to run. Start running and have your partner watch you. Throw a grenade, and as you throw the grenade you will slide across the floor. After you throw it you will resume running. You cannot notice this by yourself, as you couldn't see your own legs in this game.

Why it happens
When Bungie made the animation for throwing a grenade, they also animated the lower half of the body standing still, so while you are still running, it is not animated as such.