What is a scrum in rugby and why does it collapse so often?
A scrum restarts play after certain minor infringements — a knock-on, a forward pass, or the ball becoming unplayable. Eight forwards from each side bind together in three rows and push against the opposition, while the scrum-half feeds the ball into the tunnel between them and the hookers compete to hook it backwards.
The structure: the front row is loosehead prop, hooker, tighthead prop. Behind them the second row of two locks push through the front row's hips. The back row — two flankers and a number eight — bind on the outside and rear.
The engagement sequence is refereed by a call sequence, currently crouch, bind, set in rugby union. It was changed specifically to reduce the force of the initial hit: earlier sequences allowed a running charge into contact, which generated dangerous forces.
Why scrums collapse:
The forces are enormous. Sixteen players pushing generates loads measured in the region of a tonne and a half, applied through eight men's shoulders and necks. Small errors in body position produce large failures.
Angles and binding. Boring in — driving inward at an angle rather than straight — destabilises the structure. Illegal or short binds let a prop drop or twist their opponent.
Deliberate tactics. A dominant pack may collapse it to win a penalty, and a struggling one may collapse to avoid conceding ground. Both are penalised, and telling them apart in real time is extremely difficult.
Height mismatch. A pack driving downward on a lower opponent forces a collapse.
Safety is why it matters. Front-row play carries the highest catastrophic injury risk in the sport, which is why uncontested scrums are mandated when no trained front-rower is available.
Rugby league scrums are uncontested and largely a formality for restarting play.