found in Gampopa's A Mirror Illuminating the Oral Transmission:Sitting still in sexual union, he should create an upwards movement of his breath while meditating on the sound of a certain mantric syllable. This will give rise to four levels of joy, i.e., sexual arousal. While experiencing these, he should concentrate on the nature of his mind. Thereupon, he performs the practice of Inner Heat, whereby the wind element dissolves into the fire element giving rise to the dissolution sign of 'smoke' (rtags du ba). The white bodhicitta (byang sems, i.e., semen) then descends from the crown of his head to the penis (rdo rje nor bu, *vajraratna). At this point, the male yogi must remain in a meditative state without ejaculating any semen, thereby causing the water element to dissolve into the fire element, which produces the mirage-like sign (rtags smig sgyu lta bu). When the flow of bodhicitta has reached the very tip of the penis (rdo rje rtse mo, *vajrasekhara), the male yogi must reverse its flow back up to the crown of his head. Thereby, the water element dissolves into the earth element, causing the lamp-like sign (rtags mar me lta bu) to appear. When this experience has become very steady and the firefly-like sign has appeared, that constitutes the perfection of tranquility meditation (gzhi nas, *samatha), equivalent to the meditative concentration (*samadhi) of the first level of absorption (bsam gtan dang po, *prathamadhyana). Interrupting this absorption, the yogi should then train in insight meditation (lhag mthong, *vipasyana). That is here done by means of a visualization focusing on a small ball of light (thig le, *bindu) in the heart-cakra, which gradually turns into an experience of radiance and emptiness (’od gsal stong pa nyid). Thereby, the sign which is like a cloudless sky appears (sprin med pa'i nam mkha' lta bu'i rtags) and the yogi realizes the selflessness of all phenomena to be like the ungraspable center of the sky. Radiance yoga Luminosity, radiance or clear light (Skt. prabhāsvaratā; Tib.’od gsal) refers to the clear radiant nature the mind, which is associated with buddha-nature. It is said to be experienced during various events in one's life, such as orgasm, sleep, dreaming and in the dying - rebirth process. Tilopa's oral instructions explain this practice as follows: The yogi working with the central channel places the mind in the central channel and fixes concentration on the drop