This is a sexual act that is similar to the "69". The female participant is on top and holds her arms straight while servicing the gentlemen on the bottom. The male holds the females feet in his hands and rotates the ladies feet in a circular motion to resemble the propeller on 1850's riverboats.
David Wilcox's "Riverboat Fantasy" must be playing for the entire duration of the Upside down Riverboat.
This sexual position was inspired Cowboy's and Alien's which features an upside down riverboat.