A related practice is the visualization on the body as being hollow: "here the body and the energy channels ("nadis") are to be seen as completely transparent and radiant". This technique releases tension and gives suppleness to the subtle energy channels. Tsongkhapa describes this practice as follows:One commences as before with the practice of visualizing oneself as the mandala deity. The special application here is to concentrate on the body, from the tip of the head to the soles of the feet, as being utterly empty of material substance, like an empty transparent balloon filled with light...Here the body is to be envisioned as being entirely without substance, appearing in the mind like a rainbow in the sky.