। ॥ ३७७ ॥
In plain words
Prāṇa operates through forward motion, apāna through downward motion, and vyāna through all-pervading spread, while udāna is recognized by its function of upward departure.
Close to the Sanskrit
Prāṇa is by forward movement, and apāna is by downward movement. Vyāna is through all-pervading movement, and udāna is accepted as that of departure.