For the brain, glancing blows can do more damage

A blow to the head from an angle is more likely to cause brain trauma than one of equal force delivered squarely. Boxers know this.

“Retrospective analysis of boxing knockouts has revealed that they typically are caused by a hook to the side of the jaw which causes a rotation of the head in the horizontal plane. Uppercuts to the chin may also cause unconsciousness, while straight punches to the face are unlikely to do so”

➔ Frontiers in Neurology, “How Can a Punch Knock You Out?”

The reason has to do with the anatomy of the brain, such that a rotational or shearing force puts more stress on tissue. This is why the best helmets test and design for angular impacts. Generally, mitigation is attempted with some form of decoupling of the shell from a stable layer.

Interestingly, Mips, whose rotational mitigation tech can be found in all manner of helmets, cites a 1943 paper ➔ The Lancet, “Mechanics of Head Injuries”

“In the case of a substance such as brain, whose modulus of incompressibility (bulk modulus) is large compared with its modulus of rigidity, the amount of pulling apart of the constituent particles is proportional to the shear-strain. (Shear-strain, or slide, is the type of’ deformation which occurs in a pack of cards, when it is deformed from a neat rectangular pile into an oblique-angled pile.) Hence the’ shear-strain present at any point in the brain should be at any rate a rough measure of the probability of injury at that point.”