Deploy

Many weapons in the Big Guns category have to be deployed with D prior to their usage. Deploying a weapon costs a lot of AP - usually 80, and involves going through a short animation after which the character is visibly deployed.

This brings some balancing into the Big Guns category by having you commit to a position first before firing (Making peek & shoot maneuvers mostly an option for Small Guns which do not need to be deployed)

Being deployed gives you a small cover-bonus of 10%, 20% if you have the Barricade Tactician perk.

While deployed, any other animation will force you out of deployment and thus make you having to redeploy again and spend yet another batch of AP and time to do so. Thus, people knocking you down, knocking you out, knocking you back, or of course shoot the weapon out of your hands will cause you to lose your deployment status.