Acupressure is often called acupuncture without the needles. Instead of needles, acupressure involves the application of manual pressure (usually with the fingertips) to specific points on the body.
According to the principles of traditional Chinese medicine, the body has vital energy called "chi" or "qi" that flows along invisible lines of energy flow called meridians. There are thought to be at least 14 meridians connecting our organs with other parts of our body. Acupuncture and acupressure points lie on those meridians. If the flow of qi is blocked at any point on a meridian, it's thought to be the cause of ailments and lead to disease anywhere along the meridian. That's why a practitioner may apply pressure to an acupressure point in the foot to relieve a headache.
There is no scientific consensus on how acupressure might work. Some theorize that the pressure may promote the release of natural pain-relieving chemicals in the body, called endorphins. Another theory is that the pressure may somehow influence the autonomic nervous system.