I heard Nigel Owens talking about this on RadioSport when he was here for the All Blacks v England tests. He said that the referee needed to watch for the difference between
"aeroplane taking off" and
"aeroplane landing"
"Aeroplane taking off" = The arriving player comes in low, shoulders at least level with or above hips, binds onto the opponent (so somewhere between his hips and shoulders) and drives him upwards and away. This is a legal clean-out
"Aeroplane landing" = The arriving player comes in low, shoulders usually below hips, grabs opponent below hips (therefore not binding) or not at all, and drives downwards over the ball/tackled player. This going off your feet/sealing off. PING!!
ETA:
Check out the ARU Game Management Guidelines
http://www.rugby.com.au/Portals/18/Files/Refereeing/Laws/2012 Laws/2012 GMG 120205.pdf
Pages 2 & 3
Arriving Players
• Players who play the ball after a tackle must do so from the direction of their own goal line and directly behind the tackled player or tackler nearest to their goal line (ie. ‘through the gate’).
• Both defensive and attacking players must enter ‘through the gate’ at the tackle.
• Arriving players must demonstrate positive intent to stay on their feet.
• Attacking players must arrive supporting their own body weight and not deliberately go to ground to seal possession or deny a contest.
• A player who is supporting the ball carrier, who then latches on and goes to ground with the ball carrier in a tackle (sometimes called a ‘hammer’) must then roll away, or get to his feet and be supporting his weight on his feet. He cannot stay on top of the ball to deny a contest for possession.
• The following analogies apply to arriving players:
- "Plane taking off" is acceptable.
- "Plane landing" is not acceptable.
- "Fork Lift truck" when moving players is acceptable.
• Zero tolerance on wide hits and shoulder charges. ‘Driving out’ at the tackle must be near the ball – ie.
1 metre width either side of the ball.