Mainly hollow metal balls on a string, inserted in the rectum for sexual stimulation.
Generally hollow metal balls, attached to a chain or string, that are used for stimulation purposes, by being inserted into the rectum/vagina. Motion of hips and muscles create a pleasant stimulation feeling, to aid in orgasm.
No real proof or historical information really, other than it was found to be in Burma during the sixteenth century, and found in Thailand in the seventeenth century.
Generally small marble shaped, these devices are attached to a small chain or sting, then inserted one by one, into the rectum (or vagina) to aid in stimulation. Some can be filled with tiny weights, or vibrating metal, that helps to create the pleasurable feelings.
The role is to aid in making one achieve an orgasm, and the key is 'aid'. They are not designed nor meant to create an orgasm, merely to provide stimulation internally.
Practice (Associated Acts): There are some who believe such items aid in strengthening the internal muscles, and in providing bladder control. They have been recommended for this, not just as a sexual stimulation device.
Noteworthy: The effect of using such a device is disputed. Many sexologists claim it has very little effect, if any. However other's who use them, claim it is very beneficial in helping them enjoy the stimulation process, giving them better control over their internal muscles, and subsequently their orgasm.