A British psychologist, John Leach, has researched this subject for some years. His results may seem to indicate that the production of L Dopamine in the brain reduces and can eventually shutdown altogether.
This state however is usually produced by outside stressors, such as genuinely believing a voodoo curse, being diagnosed with cancer or AIDs, that type of incident. So it seems that this state usually occurs when the individual and unconcious mind are totally convinced that death, for them, is now inevitable and irreversible. I'm not sure how strong willed you'd require to be to "will" yourself into this condition of absolute conviction.