瑜伽師地論略纂 8.112

8.112

論云「中間入、出息」者，謂入、出二息中間，雖息未入出，其間亦有微細息轉，名中間息。

Where the treatise says, "intermediate inhalation and exhalation," it means that in the interval between the two breaths of inhaling and exhaling, although the breath has not yet entered or exited, a subtle breath moves in the meantime; this is called the intermediate breath.