You watch a screen that monitors various activities of the body like heart rate.
When a person has to deal with a lot of
pain, for example, in order to control it without drugs, they'll hook them up, and when the person sees body changes on the screen, they'll actively respond by slowing down their breathing, which alters what they see on the screen, and the forced relaxation helps pain.
In the psychological world, this is sometimes referred to as mind control.