We haven't heard that one for a while.
"Rotational forces" and "increased dimensions" (orsomething), wasn't it?
Apparently, if the outside dimensions of the head are that little bit
less because of the absence of a protective helmet, the head will
magically float above the road surface by the all important few
millimetres that the helmet is not adding and obviate all ompact and
impact injury.
It's been a few years since anyone tried that line on. The Medway
Handyman was particularly scathing about it.