This the application of a hollow instrument by creation of a vacuum in the instrument and pressing it onto the body. Traditionally this is done by placing a flame inside the cup ( or jar or section of bamboo) to burn the oxygen. The vacuum pulls the skin and muscle away from the body. I don;'t know of any other way to move these tissues in that direction other than surgery or some tuina techniques. Cupping however is not painful. The sensation is one of tightness. If the cup has too much pull and is uncomfortable its removed and place again so its comfortable. Cups are applied over acupuncture points and sometimes over needles in points. They're also applied to the meridians and spasmed or painful areas. Application is not only static but moving. An oil or creme is put on the area to act as a lubricant and once placed, the cup is slid in the necessary directions. The movement is done slowly and the practitioner varys the speed according to resistance. One technique for local pain is placing the cup away from the epicenter of the pain and moving in a spiral towards the epicenter. Most people love the feel of a moving cup. A static technique for the same pain is to place cups around the perimeter of the painful area and leave them for several minutes. Its similar to extricating a 4x4 from a mudhole. Instead of just hooking up the stuck 4x4 to another vehicle a constant tension is place on the on the winch cable which allows the mud teh time to move out of the way. When the vacuum of the mud - the muscle spasm - is defeated the vehicle can be pulled out of the mudhole.
Cups are used for quite a number of conditions in addition to pain. The picture to the left shows a common application pattern for expelling external wind.