The yogi working with the central channel places the mind in the central channel and fixes concentration on the drop at the heart. Visions arise, like lights, light-rays, rainbows, the sunlight and moonlight at dawn, the sun, the moon, and then the appearances of deities and forms. In this way the myriads of worlds are purified.Gampopa's "Closely Stringed Pearls" explains that this practice is to be done while falling asleep by initially setting an intention to "seize the radiance" ("'od gsal zin par bya"). Then the yogi visualizes five syllables in the heart-cakra. As sleep sets in, the yogi slowly shifts their attention from one syllable to the other. If one is successful in capturing the radiance or luminosity, a syllable Hum (ཧཱུྃ) will appear vividly in the heart, radiating powerful light and one will experience blissful sleep (and not the dream state, if one dreams, one has failed to capture the radiance). After waking up, it will appear as if one is in a strong glow.