How could relaxing reduce pain? Well, the uterus is built of two opposing types of muscles: longitudinal muscles, which are mostly on the top of the uterus; and circular muscles, which form the lower half of the uterus. During labor, or monthly cramping, for that matter, longitudinal muscles contract to clear things out, but if a woman tenses up, the circular muscles lower down "close the outlet, maintaining the uterus in its unemptied state. ... Then the muscles that empty the uterus and the muscles that hold it closed are working
against each other" (Childbirth Without Fear, Grantly Dick-Read, M.D., p. 67) and pain results. Just as it can become painful when the bladder needs to be emptied but the timing isn't convenient, so there is unnecessary pain when half of the muscles of the womb are trying to clear out the uterus but the other half are tense and constricted.