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The scrum formation is used to re-start play after a knock-on, a forward pass or if the ball has become unplayable.
Very often a player will lose the ball forward during a tackle or just while running and receiving a pass, thus knocking-on. If the ball is quickly picked up by the other team, the referee will let play continue to allow the recovering team to take advantage of the mistake. If no advantage occurs, then the referee will whistle for a scrum to be set at a spot he indicates on the pitch also called a mark.
The team that did not lose the ball is awarded the ball to put into the scrum. A scrum is also awarded whenever a pass is made in which the ball goes forward.
The typical procedure of scrummaging involves each set of front row players binding and the hookers calling for the locks to join the formation. The flankers join on each side of the locks setting their shoulders below a prop's outside buttock. The No.8 joins at the back between the hips of the two locks.
While this is occurring the captain of the forwards can be calling a move while the backs are shouting out code words signalling what move they will be running. The forward pack with the put in is then allowed the courtesy of initiating the coming together of the scrum. Upon a prearranged signal between the hooker and scrumhalf, the scrumhalf will roll the ball into the tunnel underneath the two locked together front rows.
Each of the hookers will then attempt to push the ball behind him with a sweep of his foot. All of this is occurring while each pack is attempting to push the other backwards driving themselves over the ball.
If the ball is won cleanly, most often the scrumhalf will run to the back of the scrum to retrieve the ball from in front of the No.8's feet and pass it to the backs, to a breaking loose forward, or make a run or kick of his own.
The opposing scrumhalf will follow looking for a chance to snap up any loose ball. The No.8 may also decide to pick up the ball himself, and start a back row move from the back or base of the scrum.
One exciting aspect of scrummaging is the pushover try. A pushover try is scored when a scrum is set close to the attacking tryline. The attacking scrum will keep the ball at the No.8's feet driving the defending pack backwards across the tryline. Once the ball has been dragged across the tryline, the No 8 or scrum-half will touch the ball down for the try.