This is called tonic immobility, a reflex that causes a hypnosis-like state of inactivity. It's a shutdown button, basically. Divers, scientists, and assholes alike take advantage of this to study and handle the animals in the wild or in laboratories. Most of the time, sharks can't get out of this state until they are turned again ... which is how they go for a little nap and wake up to find some killer whales chomping on their livers.
Yep, orcas have found a way to use this glitch against sharks. In some species, like the great white, tonic immobility is fatal, because they need to keep swimming to pump water through their gills. So, orcas use this tactic to drown them and eat their nutritious livers. All they have to do is turn the sharks upside-down, which isn't particularly hard, since sharks are usually looking up to see what they can hunt. This leaves them wide open to groups of orcas flipping them over and turning them into helpless lumps of meat.
Orcas: the rude frat boys of the sea.
This sucks for sharks, no doubt. But we were robbed of a Jaws movie in which the final confrontation is just Roy Scheider rubbing the man-eater's nose and doing some magic tricks with it like at SeaWorld.