How to do it
Go onto the Campaign level The Arbiter on any difficulty. When you get to the elevator that goes down toward the room where you battle more Elites and Grunts, throw a plasma grenade upwards. When it hits the floor of the moving elevator it should bounce up and down and not explode. (It WILL explode when the elevator stops). You can do this with as many plasma grenades as you want. This also works with weapons. To do this duel weild to weapens and dropp one abuve the evelator wile it is moving.
Effect
The plasma grenades should bounce up and down without exploding until the elevator stops moving, when it does the plasma grenades will explode.
Sources
The Arbiter (Halo 2 campaign)