Kill the Man with the Ball is a game mode in which the goal is for the player to hold onto a "ball" for as long as possible.
Players attempt to hold the "ball" (which is actually a human skull) for the longest time possible. An orange circle will appear on the motion sensor indicating the location of the person holding the ball. Weapons, including Fists cannot be used while carrying the ball, and the holder's speed is much slower than normal. The player with the most time spent holding the ball at the end of the match will be declared the winner.
Usually, the best way to deal with an enemy player is to drop the the ball and fire upon them. Pressing the primary fire button will drop the ball and the player will ready their last used weapon.