The traditional form of cupping involved heating glass cups with fire and using the heat to suction them on to the body.
Instead of pushing down on the tissue like massage, the cups actually pull up on the tissue. This helps to break up stagnation in the body and pull it to the surface to release the toxins. The more stagnation the more pain or discomfort in the body- this is where the bruising can occur. The more stagnation the deeper purple in colour the skin can turn.
The cups can be left stationary on the body or moved along the meridian lines, which is called moving cupping.