'And he who offers the third oblation, should offer it to Apâna (down-breathing), saying Svâhâ. Then Apâna is satisfied. If Apâna is satisfied, the tongue is satisfied, if the tongue is satisfied, Agni (fire) is satisfied, if Agni is satisfied, the earth is satisfied, if the earth is satisfied, whatever is under the earth and under fire is satisfied.

In plain words: The third bite should be offered to Apana, the breath that moves downward, while saying 'Svaha.' This satisfies the tongue, fire, the earth and everything beneath them.