By default the Demolition soldier does not fully make use of the heavy ballistic armor he is wearing. (Only the increased blast resistance is active by default) Triggering your Ballistic Armor lowers your face shield (the design varies between factions, though all of them restrict vision) and reduces your movement speed to a crawl, but subsequently grants you 100pts of armor on top of what your current health was. (Your health and health indicator, a " + " sign, are replaced with an Armor Indicator, a shield, and a maximum of 100pts of Armor for a fresh mask.)
In addition to showing you the numerical condition of your ballistic armor, damage is visually represented on the full face polymer masks with fractures to your face mask. (The Metal Helmets do not suffer from this, though have a smaller viewing area than the polymer ones)
Once your Ballistic Armor is depleted, the mask pops off and you resort back to your actual health. There is a minimum time frame where the player can not put on the mask after just taking it off. There is a 5 second recharge before it is available.
Soldiers using their Ballistic Armor (aka "Tank Stance") have a special icon next to their name when your cross hair is pointed at them (or they're spotted by your Buddy), and make a very distinct impact noise when shot. Explosives, Offensive Support Actions, and the "Heavy Hitter" rounds from the Pointman class are effective ways to make short use of an enemy's ballistic armor.