Verified answer

It depends on what control scheme you are using: most of them have the equipment use button assigned to the Left Bumper (the little long button above the Left Trigger with LB stamped on it), while Southpaw has that button assigned to the Right Bumper (similar as left except above the Right Trigger and stamped with RB), and Bumper Jumper has equipment assigned to the X-button.

Also, you have to have the Armor Lock ability in order to use it, not all modes have it as a possibility. If you have Armor Lock currently on you, then there should be a little symbol of a Spartan crouched down with his/her fist planted on the ground, and a little line following the outline of the Spartan.

Armor Lock gives you a few seconds of invulnerability at the expense of your mobility. If you use Armor Lock next to an enemy and then unlock while they are still next to you it acts as an EMP disabling their shield and setting them up for a quick 1 hit melee kill or headshot. Vehicles will be destroyed upon colliding with a locked person.

Ben, stop winning so hard.

I agree, look in the user manual, but for reference, its the left trigger, but it only works if you're up against a wall. Theres a glitch in the game which will make you lose health, but thats only a visual error. Activate it again for armor lock

LB