When it comes to rules and regulations, the ruck is one of the more complex parts of rugby union.
Players must be careful not to infringe the rules in a ruck
When a tackled player goes to ground, they must release the ball immediately.
As soon as that happens, the opposition will want to get their hands on the ball, and the team in possession will not want to give it away.
According to the laws, "the ruck is a phase of play where one or more players from each team, who are on their feet, in physical contact, close around the ball on the ground".
So to gain possession, both sides must try to drive over the ball to make it available for their team-mates.
HANDS IN THE RUCK
None of the tackler's team-mates can attempt to handle or pick up the ball once the ruck has formed.
Team-mates of the tackled player can use their hands, but only if they are on their feet.
Referees often blow up for penalties because a player off their feet or from the tackler's team has used a subtle hand to bring it back to their side.
But because of the sheer number of bodies involved in rucks, referees can sometimes miss this particular infringement.
JOINING A RUCK
All players must join the ruck from behind the 'hindmost' foot of the last player.
They must bind with one arm round a team-mate at the very back of the ruck.
Players cannot take shortcuts and join from the sides.
If the referee spots this, a penalty will be given to the non-offending team.
USING THE BOOT
The ball can often get stuck under a pile of bodies, making it difficult for either team to make it available.
Players are allowed to free the ball by using their boot. This term is called 'rucking'.
Dangerous rucking is an instant penalty
However, they must ensure they do not make contact with players' heads or tread on bodies intentionally.
Referees are particularly strict on this law for safety reasons, so reckless rucking can mean time in the sin-bin or even a red card.
All players joining a ruck must be on their feet.
But sometimes players dive in off their feet in an attempt to slow the ball down, allowing defences to re-group.
Referees are particularly strict on this.
BALL NOT FREE
If the ball does not come out of a ruck after about five seconds, the referee will award a scrum to the team he considers to have the greater forward momentum in the ruck.