Prayer Wheels

There are earth, water, fire and wind prayer wheels. One of the benefits of the prayer wheel is that it embodies all the actions of the Buddhas and Bodhisattvas of the 10 directions. To benefit sentient beings, the Buddhas and Bodhisattvas manifest in the prayer wheel to purify all our negative karmas and obscurations, and to cause us to actualize the realizations of the path to enlightenment. All the beings (not only the people but also the insects), in the area where the prayer wheel is built are saved from rebirth in the lower realms; they receive a deva or human body, or are born in a pure land of Buddha. If you have a mani prayer wheel in your house, your house is the same as the Potala, the pure land of the Compassion Buddha. If you have a prayer wheel next to you when you die, you don't need powa. Having the prayer wheel itself becomes a method to transfer your consciousness to a pure land. Simply thinking of a prayer wheel helps a dying person to shoot the consciousness up the central channel and out through the crown to reincarnate in the pure land of Amithaba or the Compassion Buddha.
http://www.dharma-haven.org/tibetan/benefits-of-prayer-wheels.html