20.1 (f)
In an 8 person scrum the formation must be 3-4-1, with the single player (normally the Number 8) shoving on the 2 locks. The locks must pack with their heads on either side of the hooker.
Exception: A team must have fewer than eight players in its scrum when the team cannot field eight suitably trained players in its scrum due to either the team not fielding a complete team, or a forward player being sent off or temporarily suspended for foul play, or a forward player leaving the field because of injury.
Even allowing for this exception, each team must always have at least five players in a scrum.
If a team is incomplete and it cannot field eight suitably trained players in its scrum, the scrum formation must be as follows:
If a team is without one forward player, then both teams must use a 3-4 formation (i.e. no No.8).
If a team is without two forward players, then both teams must use a 3-2-1 formation (i.e. no flankers).
If a team is without three forward players, then both teams must use a 3-2 formation (i.e. only front rows and locks).
When a normal scrum takes place, the players in the three front row positions and the two lock positions must have been suitably trained for these positions.
If a team cannot field such suitably trained players because:
either they are not available, or
a player in one of those five positions is injured or
has been sent off for Foul Play and no suitably trained replacement is available, then the referee must order uncontested scrums.
In an uncontested scrum, the teams do not compete for the ball. The team putting in the ball must win it. Neither team is allowed to push the other team away from the mark.

20.9 (j)
Maximum 1.5 metres push. A team in a scrum must not push the scrum more than 1.5 metres towards their opponents’ goal line.
Sanction: Free Kick

20.9 (k)
Ball must be released from scrum. A player must not intentionally keep the ball in the scrum once the player’s team has heeled the ball and controls it at the base of the scrum.
Sanction: Free Kick

20.11 (a)
No wheeling. A team must not intentionally wheel a scrum.
Sanction: Penalty kick
If a wheel reaches 45 degrees, the referee must stop play. If the wheel is unintentional, the referee orders another scrum at the place where the scrum is stopped. The same team throws in the ball.