Once a scrum is formed the scrum-half from the team awarded the feed rolls the ball into the gap between the two front-rows known as the tunnel. The two hookers then compete for possession by hooking the ball backwards with their feet, while each pack tries to push the opposing pack backwards to help gain possession. The side that wins possession can either keep the ball under their feet while driving the opposition back, in order to gain ground, or transfer the ball to the back of the scrum where it can be picked up by the number 8 or by the scrum-half. Officials and offences There are three match officials: a referee, and two assistant referees. The referees are commonly addressed as "Sir". The latter, formerly known as touch judges, had the primary function of indicating when the ball had gone into "touch"; their role has been expanded and they are now expected to assist the referee in a number of areas, such as watching for foul play and checking offside lines. In addition, for matches in high level competitions, there is often a television match official (TMO; popularly called the "video referee"), to assist with certain decisions, linked up to the referee by radio. The referees have a system of hand signals to indicate their decision. Common offences include tackling above the shoulders, collapsing a scrum, ruck or maul, not releasing the ball when on the ground, or being offside. The non-offending team has a number of options when awarded a penalty: a "tap" kick, when the ball is kicked a very short distance from hand, allowing the kicker to regather the ball and run with it; a punt, when the ball is kicked a long distance from hand, for field position; a place-kick, when the kicker will attempt to score a goal; or a scrum. Players may be sent off (signalled by a red card) or temporarily suspended ("sin-binned") for ten minutes (yellow card) for foul play or repeated infringements, and may not be replaced. Occasionally, infringements are not caught by the referee during the match and these may be "cited" by the citing commissioner after the match and have punishments (usually suspension for a number of weeks) imposed on the infringing player. Replacements and substitutions During the match, players may be replaced (for injury) or substituted (for tactical reasons). A player who has been replaced may