Jacking a hogtie
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.
The same effect can be achieved by arching the model's back before the ropes are tied. This is usually done by raising the thighs and putting a wedge under them while the hogtie rope is pulled as tight as possible and secured.