There was a certain Brumbies flanker that used to do this all the time. He would bind directly onto his loosehead prop in order to help him turn in on the opposing tighthead's neck. That loosehead prop was forced to retire early from the game. He had sought medical advice after experiencing numbness in his left hand and arm. In 2005, his doctors told him that if he didn't have his neck injury corrected, he could end up with a permanent disability in his left hand, but he continued to play in 2006 until a third doctor warned him against playing. He decided to retire.
Later in 2006, he underwent surgery to correct nerve damage in his neck.
I'm not telling you all this to be snarky about some Aussie rugby player. I'm pointing out that this is a dangerous technique with potential serious consequences for the prop. The fact that is gives him a clear, illegally gained advantage over his opponent is not as important as the fact that it could have serious, long term health consequences.
Referees will be doing the prop a big favour by ruthlessly penalising this action.
