The Power Drainer is a deployable device that has been seen in the leaked internal beta build Halo 3 (Confirmed in Leaked Halo 3 Zanzibar video). It has not been confirmed to be in the finalized game. The Power Drainer, like the Bubble Shield and Jump pad, is in the category of "Equipment". It can be deployed using the "X" Button.
The Power Drainer can be activated by deployment.
It can be picked up and deployed on the battlefield.
Only one equipment of each kind can spawn and be held by a player at one time.
Theory
Due to it being named the "Power Drainer" it is a device that must drain some sort of power. It looked the original Bubble Shield model (which has now been changed to something stationary). Seeing as it is a device similar to a Bubble Shield it may have a reversed effect.
Instead of keeping the player inside unharmed, it may make players outside or within to loose their own shields, making them vunerable (Instead of the protected Bubble Shield). The shields are the only power "objects" on the suit.