Each team has a skull idol in their base, and the object is to destroy the opposing team's idol. The skull regenerates, takes a crap-load of damage to destroy, and is usually heavily defended. Unless the your team is composed of guys that refuse to go on defense, Overload is a lot of fun. Coordinated attacks, and dedicated defenders are needed to be successful in Overload. Kill a bad guy, and his skull pops out of a skull generator in the middle of the map.
Somebody then has to pick up the skull, and return it to the base for the score.
Get killed with the skull in your pocket, and you lose it. Some guys I know really like this one, I personally don't.