Using what is called a dibber (a wooden stick) a person rubs around the rim of the bowl in a steady and constant motion to start the bowl singing. It will produce a note that increases in volume with every rub.
Mainly these bowls are found in places where Buddhism is also commonly found, places such as Tibet, Nepal, India, Bhutan, China, Japan and Korea. The bowls are thought to have been first used back in 900 AD to aid various types of Buddhist meditation.
Interestingly the bowls are also useful as an example of a process much like water memory. If you fill the bowl with water, rub it around the rim with the dibber, and watch the ripples in the bowl, the more you repeat the process the faster the water will show the same ripple pattern.