Structure or machine?
Is a scrum is a structure or a machine? Partly it depends whether it’s a good scrum or a bad one. The start of a good scrum should be relatively stationary, which suggests a structure.
When I think of a scrum I think of an arch bridge: the two front rows form an arch. Arches normally work by carrying load from the top out to the edges and downwards. However, in a scrum, much of the force begins from the feet of the participants.
How an arch works, how a scrum works
The thing that allows an arch to carry concentrated loads is an ability to re-channel forces in different directions, though they have to stay in the arch.
In a scrum, the arch effectively comes to a point at the top, where the two teams meet shoulder to shoulder:
If the compression gets too low, the point of contact at the top tends to move upwards – that is resisted by the necks of the men in the front row.
If compression gets too high, the point of contact wants to move down. The resistance to the drop comes from the backs of the front row, and from where they place their feet – if they can move their feet forward a little it will help. Trouble with that is, it tends to arch their backs, and if a big push comes on there is a risk of injury.
The structure of rows
In the front row the heads are under tightly linked arms. There is no real cross link at the back end where numbers four and five lock, except their own arms and those of the wing forwards beside them.
Just like a brick wall under compression, there is a tendency to split apart and that lets the second row through into the front.