The principle is that you cannot be nervous and relaxed simultaneously. Because the sympathetic and parasympathetic nervous systems are separate divisions of the autonomic nervous system. Nervous systems that unconsciously regulate opposite functions of the body cannot both be active at the same time.
“In the actual desensitization procedure, the patient is made to relax as deeply as possible and then the least disturbing scene from a hierarchy is presented to his imagination for a few seconds. Presentations are repeated until he no longer has any disturbance, and the same procedure is followed for each ranked situation up the hierarchy. There is almost invariably a transferred elimination of anxiety to the corresponding real situation. In individuals who are not disturbed by imagining situations that disturb them in reality, desensitization requires the exploitation of real stimuli, being then called “desensitization in vivo.” (Wolpe, 1968)
- Wolpe. (1968). Psychotherapy by reciprocal inhibition. Conditional Reflex, 3(4), 234–240. https://doi.org/10.1007/bf03000093