Jacking a hogtie
From wipipedia.org
Jacking a hogtie is the process of making the hogtied victim arch his or her back.
Much of the arching of a model's back in hogtie photographs is due not to the ropes but the model's ability to bend. The tying process leaves the model flat; tightening the ropes just bends the knees more and brings the ankles and wrists closer. However, if the model lifts his or her head, shoulders, and chest off the ground, the feet will rise off the buttocks with the soles pointing up, the back will curve and the hands will be well clear of the buttocks.
See also
Hogtie articles: