I still dont understand how it would work ?
A helmet increase the size of your head and bigger size dissipates proportionally more energy when you hit the water with your head.
So, your head slows down sooner, until it reaches its max allowed position.
Imagine you are doing a backward somersaults with high speed on the water.
Without a helmet your head will travel bacward of your spine, risking a brake of your neck.
It happened once again to me at higher speed.
This time it was a cream, the helmet helped a lot.
Just Keep Flying