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OK, a ref's perspective for my tuppence worth:
Firstly, U/19 call was crouch, touch & hold, engage. That's 3 calls. The new call is crouch, touch, pause, engage; 4 calls. This is, as I understand it, for all grades of community rugby. As well, obviously, as the S14 & international stuff on the telly.
At the "touch" call, the props touch AND THEN DROP THEIR ARMS, or retract them. They don't touch and get a grip on the other prop's shoulder or any other body part. Obviously it is a safety issue, as the ref is the one with the duty of care to the players (particularly at scrum time). I blame the goddam yanks, and their lawyer programs, who show us colonials that, no matter what an individual does wrong, they can almost always sue someone else for letting them do it. (apologies there to any American folk reading this - it's just an expression ....)