This is the name of a sexual position which is both very stimulating for both the male and female. You enter from behind as you would in "doggy style" but then, the female lies on her belly with her legs together. You lie on top and continue to thrust as normal.
It is named the accidental because you know whoever invented it did so because himself and the female fell over doing doggy.
"Lets do doggy"
(both fall over)