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 get more Elites and Grunts, throw a 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 switch a weapon while the elevator is moving.
Effect
The plasma grenades should bounce up and down without exploding untill the elevator stops moving, when it does the plasmas will explode.
Sources
The Arbiter (Halo 2 campaign)