Not P but DFW: forked dorsally

[Mike Jing]

The following excerpt is from David Foster Wallace's *Big Red Son *(from*
Consider the Lobster and Other Essays*):

A second-tier Arrow Video starlet in a G-string poses for a photo, forked
dorsally over the knee of a morbidly obese cellphone retailer from suburban
Philadelphia. The guy taking the picture, whose CES nametag says Hi and
that his name is Sherm, addresses the starlet as “babe” and asks her to
readjust so as to “give us a little more bush down there.”

What exactly does "forked dorsally" mean here?