A defending player kicks the ball in touch on the full from their own in-goal area
Every member of the defending team must be behind the kicker and the tryline when the kick is taken.
The ball must travel at least 10 metres forward.
The attacking team are pretty much guaranteed the ball every time, so the advantage is entirely with them.
For example, if the attacking team puts a clever kick behind the opposition's defence, a defender is often left with few safe options.
Surrounded by attackers looking to score a try on their goal line, their best bet is to ground the ball and relieve the pressure - but only for a short time.