An impact to the head will involve both linear forces and rotational forces.
Linear forces are direct straight-line forces that compress or stretch the brain within the skull. Rotational forces are angled forces to the head from a ball, head, elbow, knee and ground. Rotational forces twist and shear the brain and brain cells.
Nature did not design the brain for rotational forces and injury, with the brain being badly designed for sudden rotational acceleration and deceleration. In sport, the brain is subjected to sudden acceleration and deceleration, because it is rotated from impacts at the side and back of the head e.g., head-to-head, head-to-ball, head-to-ground contacts and hits.
The white and grey matter of the brain is made up of differing densities, which means the jelly-like brain sections move at different speeds when rotated. This causes twisting of the brain, shearing of brain cells, brain cell death and the disruption of the nerve cell connectivity networks in the brain.