From Halopedia, the Halo wiki
Armor Lock, sometimes referred to as Armor Lockup, refers to a form of energy shielding technology. Armor Lock envelopes its operator into a powerful energy shield, rendering them virtually impervious to all damage for a short period of time. As its biggest drawback, the device also generates a local gravitic effect that immobilizes the operator. After a five-second period of continuous use, the device deactivates and releases an electromagnetic pulse.
Originally developed by the Covenant and later reverse-engineered by the United Nations Space Command, both versions of Armor Lock are either enabled by an external "plug" attached to the back of the operator's armor, or are integrated to the armor's systems and operate without an external device of any kind. The UNSC version of Armor Lock, which was at its prototype stages by mid-2552, has internal components virtually identical to the Covenant's. By 2552, Armor Locking technology was utilized in the battlefield by Sangheili and Jiralhanae combatants exclusively in the Fall of Reach, as well as SPARTAN supersoldiers.
Armor Lock is one of the seven Armor Abilities in Halo: Reach. When performing Armor Lock, the player is impervious to all damage on their health and shields, but renders the player immobile. When Armor Lock is activated, incoming rockets will be deflected while vehicles attempting to ram the Armor Lock user will be damaged or destroyed. The animation for a SPARTAN activating Armor Lock is the user slamming his/her fist on the ground. When Armor Lock is activated, an electromagnetic pulse is generated, which is emitted when the user deactivates Armor Lock. The longer Armor Lock is activated, the stronger the electromagnetic pulse emitted will be.
Armor Lock can be charged up to three stages, with each stage inflicting a stronger EMP and pushing opponents farther away. Health and shields restore normally while Armor Lock is in use. Like all other armor abilities, Armor Lock will recharge once you stop using it. A quick way to neutralize an Armor Locking player is to remain outside the EMP zone, and hurl grenades timed to detonate just as the player's Armor Lock deactivates.
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. To prevent an Armor Lock user from simply being gunned down as soon as they exit 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.
As of the final 2011 title update, Armor Lock energy will decrease when getting attacked at a rapid rate. Also in the title update, Armor Lock will not shed plasma grenades stuck on the player.
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