大毘盧遮那成佛經疏 17.27

17.27

In the fourth month, one is in the wind circle, which is a crescent moon (the same as above), black within. One takes the syllable HA as one's body, with the in-and-out breath and so forth as explained above. In this one month, one abides drinking only wind, eating no food whatsoever. This drinking of wind also means taking the in-and-out breath of the syllable HA as nourishment, not like non-Buddhists who live by ingesting air. One should form the wheel-turning mudrā; this is the mudrā with hands clasped in reverse formed previously. This is difficult to perform.