大毘盧遮那成佛經疏 17.27

17.27

第四月在風輪中，是側月也（與上同），其中色黑。用訶字為身，出入息等如上說。此一月中但飲風而住，不食一切食也。此飲風，亦是以訶字出入息為食，非如外道飲氣而生也。當作轉法輪印，此即是反手相叉，前所作者也。此難作者。

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.