As a subsidiary effect, bicycle helmets also spread point impacts over a wider area of the skull. Hard shell helmets do this better, but they tend to be heavier and less ventilated, so they are more common among stunt riders than road riders or mountain bikers.
Helmets are most effective in straight line, or linear, blows to the head at moderate speed. Helmets are not well designed to deal with high speed impacts or rotational stresses (crashes that are not centered, and crashes that involve rotation of the head). They are also not designed to provide adequate protection
for a collision involving another moving vehicle, (e.g. a car).
The helmet should sit level on the cyclist's head with only a couple of finger-widths between the eyebrows and the helmet brim. The strap should sit at the back of the lower jaw, against the throat, and be sufficiently tight that the helmet does not move on the head. It should not be possible to insert more than one finger's thickness between the strap and the throat.