A sawhorse is basically a beam with four legs. It is traditionally used to support a board or plank for sawing. In the context of BDSM, it has been perverted to be used either as a compact whipping bench or as a torture device in its own right.
For use as a torture device, the victim is made to straddle the sawhorse. The person's weight then rests on the beam between their legs. After a short time, the pressure from the beam will become very uncomfortable. This is often used as a form of predicament bondage whereby the victim is forced to stand on tiptoe to keep their weight from resting on the beam. As their feet tire, they are forced to sit on the beam causing discomfort. When the pain from the beam is too much, they raise themselves off the sawhorse again until their feet tire and the cycle begins again. The width, shape, and presence of padding will greately effect the length of time the victim can be bound in this manner.