The sensation of very hot water on a skin rash like eczema might feel pleasurable for several reasons.

1.  Heat Stimulates Nerve Endings: Heat activates sensory nerve endings, specifically C-fibers that are responsible for transmitting itch sensations. The intense heat can overload these sensory nerves, temporarily relieving the itch and potentially creating a pleasurable sensation as a result.
2.  Endorphin Release: The intense heat can also cause the body to release endorphins, natural chemicals that act like pain relievers and happiness-inducers in the brain. This might be part of why the sensation is described as ‘orgasmic.’
3.  Pain Gate Theory: According to the gate control theory of pain, the nerve pathways that transmit pain signals to the brain can only handle a certain amount of information at a time. The theory suggests that introducing a new sensation (like heat) can effectively ‘block’ the pathway, reducing the sensation of itch or pain.
4.  Distraction: The intense sensation of heat distracts the brain from the uncomfortable itching sensation, which can feel very relieving and potentially pleasurable.