A cantilever is an inside edge spread eagle executed while leaning backwards forcing the edge to glide in direction of an outside edge spread eagle. This trick was made famous by Werner Groebli aka Mr. Frick. The secret to this trick is having a decent spread eagle, strong abs, and no joints. Sitting down in an inside spread eagle is a good place to begin. From there lean back so that you begin to force the edge the other way with your weight.