The condition known as subspace is caused, as explained to me by a neurologist friend of mine, by endorphins, which are released into the bloodstream in moments of acute physical stress or pain. This endorphins block first the pressure receptors on the skin and then the pain receptors in the brain. This is why the sub stops feeling sharp pain before experiencing that dreamy, "I'm flying" type of sensation.

Depending of what else is in the bloodstream (alcohol or drugs), it is possible to experience delusions or halucination, but the natural feeling is one of euphoria. If undergoing a very intense session, the sub may actually overdose on her own endorphins, causing the shock symptoms of contracted pupils and shivering, as well as impaired vocal skills.

Of course that is just the physiological angle. On a mental level, at least in my case , it causes what I call a 'mental orgasm' of bone-liquifying intensity. It gets me to that point where I can see my skin breaking under the lash and my only reaction is "Yes... I want more"