Sharks typically enter a natural state of paralysis when inverted. This state is called "tonic", and the shark can remain in this state for up to 15 minutes, Tonic immobility (TI) is an unlearned behavioral response characterized by a state of immobility and torpor.
Some scientists relate it to mating. Sharks may not always respond to tonic immobility by physical inversion of the animal, as has been demonstrated with lemon and reef sharks. With tiger sharks 3-4 metres (10 to 15 feet) in length, tonic immobility may be achieved by placing hands lightly on the sides of the animal's snout approximate to the general area surrounding its eyes. Great White sharks have been shown to be not as responsive as other species whenever tonic immobility has been attempted. Scientists believe that tonic immobility, displayed by sharks, may be linked also with defense, because female sharks seem more responsive than others. During tonic immobility, the dorsal fin is traighten, and both breathing and muscle contractions become more steady and relaxed.
When you turn a shark upside down like this it's completely odd. It's an odd posture that they never get into normally.
Now once she's in Tonic Immobility, her visual fields are reversed and her gravity is reversed and I think this produces a kind of a sensory overload and the brain secretes serotonin and that is what's correlated with these states of immobility.

Scientists have used this phenomenon to study shark behaviour. Using the inmovility is easier to handle the animals to carry on studies on new shark's repellents that would save thousands of sharks, by appliying these repelents to hook lines, and fish nets.
Tonic inmovility is a way to demostrate sharks don't deserve their bad reputation, if done carefully by a professional, it doesn harm the animal at all, but actually proves and educates people of friendly behaviour of this animals.