From Halopedia, the Halo wiki
The Overheat Grenade Glitch is a button combo.
You must have a plasma weapon that can overheat and at least one Plasma Grenade. When you overheat plasma weapons the following glitch will apply, example given for plasma pistol.
When you release a Plasma bolt after the pistol is charged and released, you can hit Y and L quickly after each other and the Grenade will fly without its throwing animation. It especially helps if you missed the plasma shot, because your opponent will most likely take cover behind a wall. After the grenade is thrown, there is a small time you cannot shoot, but doing a cancel reload (Y, Y) fixes that.
Known to work with both plasma rifles, the plasma pistol, and the Beam rifle.
Note: This will not work on Halo 3.