It is supposed to be a spiritual experience as in the "sun ritual" performed by certain American Indian Tribes. It causes the brain to release intense combinations of endorphins that lead to an altered consciousness. It is so intense that early white Americans watching the ritual actually decided they had to ban it's practice. They were mainly appalled that the ritual involves actually piercing behind the pectoral muscle and then tearing the hook out of the flesh to end the ritual...the participant literally hangs until they fall or in other rituals balances on his (this is usually a male ritual though in the BDSM scene women also participate in the practice) tip toes until the hooks give way. The participant allows their consciousness to shift and then experiences vivid spiritual vision journeys. This is in most tribes a sacrifice along with a vision quest.
Similar ritual are enacted in other parts of the world, most involving piercing the skin with metal, bone or wooden objects often leaving no scarring or actual evidence of the ritual.

I've witnessed the ritual aspect of hook suspension and the piercing/tatooing rituals and they do seem to induce almost a state of euphoria in the participants. Most people these days do not actually tear the hooks from their flesh but it is a rather intense sight even still.