This issue came up awhile ago in the paper, but I found the description on the site a bit more accurate to me:

Of the six positions, the "freefaller" is the more rarefied of sleeping shapes, with just 6.5 per cent of people preferring to sleep on their front. They tend to have "a brash and gregarious exterior", although this confident front hides a nervous personality that responds badly to personal criticism.

You can read more here.