300G is a considerable amount of force. The foam inside a helmet is design to crush to slow down the deceleration process. Part of that process includes crushing of the foam structure, and in many cases, disintegration of the foam.
If anyone (except the hardcore disbelievers) has any doubts whatsoever about the effectiveness of this, do what I did regularly with kids -- get two watermelons about the size of a human head. Dress one in a helmet; leave the other bare; funny faces are optional. Lift both to chest height and with the helmet pointing towards the ground. Drop. The result is pretty dramatic, including the spread of red bits all over the place. Guess which one the red bits come from.