Tonic immobility is a natural catatonic state that some animals enter into when they perceive a threat. By becoming immobile they hope to avoid attracting attention from a possible predator. Sharks when inverted onto their backs enter this state but it is unknown why. It is possible it disturbs their balance or some electrical field within their anatomy. Orcas have been know to use this to their advantage. Witnesses have seen an Orca ram a Great White Shark dazing it before maneuvering the shark onto its back inducing tonic immobility and rendering it helpless to the attack. Orcas have also been filmed off the coast of New Zealand feeding on stingrays using tonic immobility.
Divers experienced with sharks have learned that rubbing the nose of a shark is evidently a pleasurable sensation that elicits a calming state in the animal. While this should not be confused with the catatonic state of tonic immobility it does allow the diver to create a trance like state in the shark. Once sufficiently relaxed the diver can then position the animal at any attitude, from standing the shark on its nose to fully inverting it on its back to induce tonic immobility.