Anticlastic Raising is a technique of metal forming whereby sheet metal is formed directly with a hammer on a sinusodial (snake-like) stake. A flat sheet of metal is shaped by compressing it's edges and stretching the center so that the surface develops two curves at right angles to each other. The pattern of the sheet plays a major role in the form that will be achieved, however, many different forms can often be made from the same pattern.
Here is a short video of Michael Good on Anticlastic Raising. Enjoy.