I'm pretty sure that they're referring to your shooting stance while indoors. Without having too much training in the area, I've heard of a technique that requires the weapon positioned perpendicular to your shoulders, coming right off your centre line. The purpose for this is so your weapon is pointed right in your direction of travel and will therefore enable you to engage close range targets quickly, while moving through a breached door and into the room. I'm pretty sure this is a SWAT technique that isn't taught to soldiers on a large scale.
Outdoors, I can't see why you'd want to present a larger target to the enemy with or without body armour. Firm girp, steady position, and proper alignment take priority over body armour that won't save you from machine gun fire IMHO.