Its called tonic immobility. With reef sharks you grab them by the tail and turn them up side down and they go tonic. You can do it to tiger sharks too by placing your hands on either side of the snout, basically over the lorenzini ampules (sensory pits in the sharks snout). It works for whites too but not as well. We only ever did it to reef sharks at uni.

This guys a bit of a tool but the vision of him taking a fin ride on a great white is rad.