Wikipedia
A ball-up (pl. ball-ups) in Australian rules football describes a method of restarting play at a neutral contest after a stoppage within the field of play. It is not to be confused with a boundary throw-in which occurs in most cases when the ball is forced out of bounds. Ball-ups may be considered to be the equivalent of a jump ball, faceoff or dropped-ball. Historically they were performed in most cased by a field umpire throwing the ball firmly into the ground such that it bounces directly upwards several metres into the air. They are also known as bouncedowns.
The Australian Football League amended the rules prior to the 2013 season to instruct the umpires to only bounce the ball at the start of each quarter and after a goal is scored, and throw the ball vertically upwards at all other times. Previously throw-ups were only used if ground conditions did not allow for the ball to be bounced effectively, or after a bounced ball-up is poorly executed and recalled (skewing in such a way that only one of the competing ruckmen is able to contest the ball). The skill of being able to bounce an oblong shaped ball vertically upwards on a grass surface is unique to Australian Football and the difficulty has been blamed for deterring some from becoming umpires.
The ball-up executed in the centre circle at the beginning of each quarter and after each goal is known specifically as the centre bounce. Centre bounces are contested by one nominated ruckman from each team, who attempts to direct the ball to a teammate.
Ball-ups can also occur at other locations around the ground. Umpires often have to decide whether a player tackled while in possession of the ball had a prior opportunity to pass the ball before they were tackled; if so, they will pay a free kick for holding the ball. If there was no such opportunity, then the umpire will call for a ball-up. Ball-ups are also frequently called when players from both teams scramble for a loose ball and neither can clearly secure possession. At ball-ups which are not centre bounces, other players may help their ruckman in the contest by unexpectedly entering the contest and becoming the third man up.
A ball-up is also called for if a defensive player kicking in after a behind has been scored moves outside of the goal square before kicking the ball. In this case, the ball is bounced at the top of the goal square.