The Scrum - key aspects
The most decisive factor: the side that gets the better hit is generally the one that wins out. An age-old tactic is to drop the scrum if you are on the back foot after the hit to get another go at it. This carries the risk of being penalised but referees will often allow a reset initially. The 'crouch, touch, pause, engage' rule has brought the scrums closer together to reduce the risk of injury as well as delaying the engagement process. The result is a huge increase in free-kicks for early engagement as teams seek to gain hit advantage.
Cohesion is critical at scrum time and any looseness between the front and second-rows can be disastrous. The back-row, particularly flankers poised to break away to the next phase of play, tend to be the greatest miscreants when it comes to binding, but it is an essential part of the operation. The tighter the better. When it comes to binding on the opposition, there are strict laws: props cannot grip the chest, arm, sleeve or collar of the opponent or exert any downward pressure.
THE EIGHT-MAN SHOVE
The best scrummaging units draw energy from every member of the pack. It depends on timing and practice, with specific calls for the hit and secondary shoves. A smaller pack can still form an effective scrum if they can synchronise their efforts from 1-8. Watching the No 8, who should be bound on and driving through the second-rows when engagement occurs, is instructive in this regard. Body position is essential, low with a straight back -- if your backside lifts or your back curves it is all over.
There was a time when hookers would spend hours after practice leaning against a goal-post practising their striking. Not so much these days. It is still an important skill -- a clean strike increases the quality of possession and the hooker must also be able to direct it to channel one (through loose head's feet and between the loose-head flank and second-row) to get the ball away quickly or channel two (slow ball back to the No 8 in between the second-rows). However, in the modern game, referees are indulgent when it comes to crooked feeds and scrum-halves get away with delivering the ball into the second-row, diluting the art of striking and eliminating the possibility of a strike against he head.
There are numerous tricks and techniques used to gain an advantage at scrum time. If the scrum is under pressure, the deliberate wheel is often employed to turn the scrum 90 degrees, as the put-in is then switched over. Other ploys include tight-heads twist inwards to attack the hooker and prevent the loose-head getting underneath; loose-heads and hookers combine to attack the tight-head and, for more old-school operators, there was the second-row punching the opposition hooker and props deliberately growing stubble to rub against opponents.