From Halopedia, the Halo wiki
The grenade lunge is a trick that can be performed in Halo 2.
Lunge at an enemy, then, right before you hit them, throw a grenade. The plasma grenade will always land on them. Immediately jump backwards, or the explosion from the grenade will kill you, too. Another way is to throw a sticky grenade if you don't have time to respond to a attacker, take them down with you so the kill they would have gotten would be equaled. Note that this fails on certain enemies, such as Tartarus, who cannot be stuck. Sometimes, even, you will be stuck when the grenade bounces off. In addition, you will be brought to close proximity of the weapon of whom the grenade bounced off (ex. Gravity Hammer).