There are many laws within rugby union, and some are harder to understand than others. A common misunderstanding within the game is the difference between 'pushing' an opponent, which is allowed, and a 'knock on', which is not.
Centurion provides a simple explanation below to some of the more complicated rugby laws.
Players can only tackle a player in possession of the ball. If a player deliberately obstructs the way of his opponent, because he thinks is rival stands a better chance of getting to the ball first, this is disallowed.
Charging and pushing - you cannot charge or push an opponent when running for the ball unless the contact is shoulder to shoulder.
Blocking - if you deliberately block an opponent to try to stop them from tackling one of your team-mates or you try to shield a team-mate, this is also disallowed.
Standing - if you stand in a position which could stop an opponent from playing the ball this is counted as an obstruction.
Running - players who are running with the ball after it has left a set-piece cannot make contact with a team-mate in front of them.
Flankers - cannot block the opposition's scrum-half whilst they are advancing around the scrum.
All of the obstruction offences outlined above are awarded with a penalty to the opposition. The offender could also find themselves shown a yellow card which could accumulate to 10 minutes in the sin-bin.
Knocking & Passing laws:
Rugby union is one of the only ball games where a ball is not allowed to be passed forward. If a player is moving towards the opposition's dead ball line they must pass the ball to one of their team-mates either along or behind an imaginary line known as the 'dead ball line' that runs right angles to the side of the pitch.
Knock on - If a player fails to catch or pick up a ball that has been passed cleanly, for example, if it travels forward off a hand or arm and hits the ground or another player, this is called a 'knock-on'. But if the player manages to catch the ball before it hits the ground, it is not a knock-on. If a knock on occurs, the referee will stop play and award the scrum to the team who has not knocked on.
Knocking with intention - if a player deliberately knocks or throws the ball forward, a penalty is awarded to the opposite team. If the referee feels a try could have taken place if the ball had not been intentionally knocked, a penalty try is awarded.
Charge-down - the one exception to the knock-on rule is the charge-down. If a player charges down the ball as an opponent kicks it, it is not a knock-on, regardless of whether the ball travels forward.
Line-out - if the ball is thrown forward at a line-out, a scrum is awarded 15 metres from the touchline.
Failure to pass - If players are not passing the ball, or a player is tackled and the ball goes forward, this is also disallowed.