What is a Knock-on in rugby?

A knock-on occurs when a player accidently knocks the ball forward with hand or arm. A scrum will be called by the referee and the team not knocking on will have the advantage of putting the ball into the scrum.

If the knock-on is considered deliberate by the referee he will order a penalty kick against the offending side.

Sometimes the referee will allow play to continue after a knock-on if he thinks the side not knocking on has gained an advantage from their opponent's error.