Armor lock

The unnamed shielding attachment is an external energy shield generator, developed by the UNSC as support for the shielding system of the Mark V MJOLNIR Powered Assault Armor. The use of this device is referred to as "Armor Locking".

Description
This device can temporarily enhance a SPARTAN's energy shield into near-invincibility, although when the shield is active, the user is unable to move. After a five-second period, the device deactivates and an electromagnetic pulse is released from the armor. The Covenant also possess a form of this technology, but the means through which it is enabled is unknown.

Armor Locking
It was previously thought that the device was only available for those playing Spartans in Halo: Reach. However, it has recently been revealed that there is another version of Armor Lock for Elites, though whether this extends to multiplayer is unknown. When performing an Armor Lock, the player is impervious to all damage, but it also renders the player immobile. When Armor Lock is activated, it will shed stuck Plasma Grenades and Needler rounds, Plasma Launcher rounds will be rendered harmless, and rockets will be deflected. It will charge up a powerful Electromagnetic Pulse when in use, which is detonated when Armor Lock is released. The animation for Armor Lock is the user slamming his fist on the ground. In doing so, he starts to emit an EMP as long as the button is held or until the Armor Lock ability is over.

Armor Lock can be charged up in three stages, with each stage inflicting a stronger EMP and pushing opponents farther away. Health or shields restore normally when Armor Lock is in use unless someone attacks you while in Armor Lock, in which case the recharge timer is reset just as it does when the player takes damage normally. Like all other Armor Abilities, Armor Lock will recharge once you stop using it. A quick way to neutralize the advantage of an Armor Lock-using player is to remain outside the EMP zone, fire one shot at him or her every once in a while to keep their shields down, and then attack when he or she either releases the button or is unable to maintain Armor Lock

Players who try to melee an Armor Locked adversary are momentarily stunned, allowing the Armor Lock user to gain a small advantage over them if they exit Armor Lock quickly enough. Also, as a means to prevent an Armor Lock user from simply being gunned down as soon as they exited Armor Lock before they had a chance to defend themselves, there is a quarter-second interval after Armor Lock is released in which the user remains invulnerable - enough time for a player to exit Armor Lock and take a single step in whatever direction they want to go before becoming vulnerable again.