Reactive Armor

Reactive Armor is a type of vehicle armor addition that reacts in some way to the impact of a weapon to reduce the damage done to the vehicle being protected, by creating a force which is directed oppositely to the penetrating round and stops the round or makes it slower and less damaging.

Battlefield 3
Reactive Armor is a specialization for tanks and other ground vehicles. When equipped on tanks, three panel icons appear on the sides and rear of the tank icon, representing additional protection from certain weaponry. If a panel is struck, it is sacrificed to negate damage inflicted on the vehicle itself. Like other specializations, it is only active if the driver has selected this gadget and once it was used / was destroyed, it takes time to recharge/reactivate the gadget. ( For better undestanding use this: A enemy rocket hits your back, the reactive armor is destroyed and absorbs all damage. After a certain amount of time (2 or 3 minutes - i'm not sure) it gets up again, and can catch the next rocket in your back.

And no engineer can repair this, because its a gagdet with its own cooldown mechanism.

As a site note this doesn't deliver protection to the front side.

In this video you can see a player using this gadget.