After the suicide of his wife, Wim decided to use his grief as motivation to learn extreme levels of self-control. Through meditation and breathing techniques, Wim is able to control his physiological functions to withstand extreme temperature, pain, and even control the body's autoimmune processes.
He has been extensively studied by researchers, and was even injected with an endotoxin- which his body was able to fight without the normal elevated, inflammatory immune response.
Wim is a living example of the ability for the power of the mind to overcome the perceived physical limitations of the body.
Here is a video of a TED talk he gave outlining his approach.
His method resembles the Tibetan meditation technique of "Tummo" or "Inner Fire". Using this technique, Buddhist monks living on the chilly plateau of Tibet would stay out in the elements overnight, wearing nothing but a wet cloth.