What's the difference between helmets and how do you find the right fit for you?:
There are two basic types of helmets. Single impact and multi-impact helmets.
that the material has little to no rebound. So while still better
than nothing, if you crashed and hit your head on the exact same spot,
there would be no material to absorb the impact. Single impact
helmets manage a MAJOR crash better than a multi-impact helmet but
then are rendered useless.