To understand the mechanism of how magnets affect the body and how in particular they will affect a painful area, we should understand how they work. Magnets come in different strengths. The strength will be referred to as Gauss . In other words the magnet may be 500 gauss to 3000 gauss. To simplify an explanation, like poles repel and unlike poles attract.
Lines of force are three dimensional, surrounding a bar magnet on all sides.
When opposite poles of a magnet are brought together, the lines of force join up and the magnets pull together. When like poles of a magnet are brought together, the lines of force push away from each other. Our bloodstreams contain positively and negatively charged ions. By applying the magnets to the painful areas, changes take place in the chemistry of the blood by affecting the ions. This causes vasodilatation thus increasing the blood flow.