瑜伽論記 7.35

7.35

息有二行者，入息下行以從鼻口入下向齊處故，出息上行以從齊處上趣口鼻故。

Regarding 'the breath having two movements': the in-breath moves downward because it enters from the nose and mouth and goes down toward the navel; the out-breath moves upward because it goes up from the navel toward the mouth and nose.