Rotational Acceleration

The brain floats in cerebrospinal fluid inside the skull. A linear impact, like a push straight backward, moves the skull and brain together, and the brain tolerates that reasonably well. Rotational acceleration is far more dangerous.

When a punch strikes the jaw from the side, the head snaps into rotation. The skull turns, but the brain lags behind in its fluid. This creates shearing forces that stretch and compress neurons unevenly. The brainstem, which controls consciousness, sits at the axis of rotation, so it is stressed disproportionately.