The Forerunners constructed massive plug locks that are required to be disengaged in order to deactivate the Containment Shield in the Sentinel Wall.
All four plug locks have to be disengaged or the Shield remains activated. The plug locks are well defended by Sentinels and Enforcers to keep the locks from being disengaged beyond orders.
To disengage the plug locks, they must be struck or hit. Alternatively, one can "press" them if playing on the Easy or Normal difficulties.
Plug Locks look very similar to Pistons, and they, like Pistons, only appear in the level Sacred Icon, in Halo 2.