The prayer wheel is a cylinder on a spindle containing a tightly wound scroll printed with a mantra many thousands of times over, and turning it is held to have the effect of reciting the mantra it contains.
Turning as reciting
The cylinder is filled with a paper scroll on which a mantra, most often the six syllables of the mantra of the bodhisattva of compassion, is printed over and over, and the wheel is turned clockwise, each turn understood to send out the accumulated recitations and the merit and compassion they carry. The practice makes the repetition of the mantra a physical act available to the illiterate and the busy alike, and it may be done while walking, waiting or reciting aloud, so the wheel multiplies devotion in a form anyone can perform.
From hand-held to river-driven
Prayer wheels range from small hand-held cylinders spun on a weighted handle to large drums set in rows along the walls of temples, turned by pilgrims as they pass, to great wheels driven by wind or by the flow of a stream, which are held to turn the mantra continuously without human effort. The device is characteristic of Tibetan and Himalayan Buddhism and belongs to the same devotional world as the mantra printed on flags and carved on stones. Its logic, that the mantra acts through the turning as through the speaking, is internal to the tradition and is the assumption on which the whole practice rests.