Visualization of the Wheel Called Om Mani Padme Hung

The visualisation of the wheel called Om Mani Padme Hum comes in the tantra "The Condensed Secrecy of the Compassionate-Eyed One". Beams from the great Dharma Wheel take all the delusions, sufferings, seed, and imprints of all six types of sentient beings. All these are absorbed into the six-syllable wheel. All the disturbing thoughts and karma, seed and imprints of the six types of beings are burnt destroyed and purified. If one does this practice one is liberated from the cause of samsara, the condition and result: all the disturbing thoughts, karma and obscurations. This is the wheel cutting the root of samsara. keep extremely secret and cherish this practice.
Beams emitted away from the mantras of the precious wheel strike the migratory beings, spontaneously actualizing the four immeasureable thoughts - loving kindness, compassion, joy, equanimity - and completing the six paramitas - charity, morality, patience, perserverance, concentration and wisdom.
Vajrasattva said, "By turning this wheel called Om Mani Padme Hum, some beings will experience the first bhumi, Extreme Joy. From there up to the tenth bhumi, Clouds of Dharma, the ten bhumis will be completed. First one completes the five paths, from the path of merit up to the path of no more learning, and then one becomes enlightened spontaneously with the resultant five bodies and five wisdoms."
In "Chakra of Wind" Master Nagarjuna says, "A being who is either touched by the continuum of wind from the wheel or by the wheel's shadow will be liberated."
Master Padma [Padmasambhava] also says, "The purposes of this wheel are as follows: even those lacking perseverance in their practice, who pass the time passively, will be able to attain siddhis by relying on the wheel. While those with perseverance will be helped in verbal recitation. Even enormously sinful actions can be purified effortlessly. By pronouncing and writing [the mantra] one can purify all faults and attain all excellent qualities. By gathering under the shadow of the wheel, migrators with the eight fetters can be liberated. By mere contact with the four elements that have activated the wheel, one will be freed from lower rebirths. In short, all unwanted faults can be purified without any effort. Just contacting the wheel once can cause one to achieve liberation and all excellences effortlessly."

Prayer Wheel Visualizations
(Extracted from a discourse given by Ven. Lama Zopa Rinpoche at Land Of Medicine Buddha, July 1994)

There are two visualizations.

With the first, you visualize the light beams coming from the (Om Mani Padme Hung) mantras in the prayer wheel, illuminating you and purifying you of all your disease and the causes of disease: your negative thoughts and the imprints of those left on your mental continuum (alaya). You then visualize the light illuminating all sentient beings and purifying all of their sufferings, as well as their negative karmas and obscurations.

With the second visualization, beams are emitted from the mantras and, like a vacuum sucking up dust, they hook all the disease and spirit harms and, most importantly, the causes of disease: the negative karmas and obscurations. All these are absorbed or sucked into the prayer wheel. While reciting five or ten malas of the mantra, you visualize purifying yourself in this way. At the end, recite some malas while visualizing that the beams emitted from the prayer wheel purify all the sufferings and obscurations of the sentient beings of the six realms. These absorb into the prayer wheel and all sentient beings, including you, are then liberated, actualizing the whole path and becoming the Compassion Buddha. (You can also do circumambulations with the same visualizations).

Advice from Rinpoche:
"If someone with AIDS, cancer or some other disease meditates like this every day, for as many hours as possible , there would definitely be some effect. Even if a person does not know about Dharma, about reincarnation and karma, if they want to have peace of mind now and a peaceful death, if they care about having a healthy body and a healthy mind, they should use this extremely powerful and meaningful method of healing. I WOULD LIKE TO EMPHASIZE THAT EVERY LARGE AND SMALL PRAYER WHEEL CAN BE USED BY SICK PEOPLE FOR HEALING. This practice is very practical and meaningful."

It's been recommended by High Lamas to expand Bodhicitta visualisations to all galaxies.