Awarding
A scrum is awarded in a number of situations. The most common is if the ball has been knocked on (knocked forward) or passed forward. The two other major times when a scrum is called for is when someone is accidentally offside; or when the ball is trapped in a ruck or maul with no realistic chance of it being retrieved. The scrum feed goes to the team that did not offend, except in the case of the ball being trapped in a ruck or maul. In this situation the feed is awarded to the team that was "going forward" — that is, the team that had the forward momentum at the ruck. If the ball is held up at a maul, the scrum is awarded to the team not in possession when the maul started.
If a penalty is awarded for a more serious offence, the team to which it is awarded may elect to have a scrum rather than take a penalty kick or free kick. This is usually happens when the attacking team is close to the opposition's goal-line, and wants to occupy all the opposition's forwards in one area to give the backs more space. They may also think they can force the scrum over the goal-line and score a "pushover" try.
A pushover try can occur the instant the ball touches the goal line. This is because a scrum is defined in the Laws of the Game to only exist within the field of play and not within the in-goal. Hence as soon as the ball reaches the line, the scrum ceases to exist and any player can ground the ball without risking a penalty for "handling in the scrum".
Pushover tries are rare, however, because scrums are awarded a minimum of five metres (16.4 ft) from either goal-line, and either touchline. This means for a pushover try to occur, the scrum must be forced several metres in one direction without collapsing.
Read more about this topic: Scrum (rugby Union)