Shatapatha Brahmana 13.2.8

Now the gods, when going upwards, did not know (the way to) the heavenly world, but the horse knew it: when they go upwards with the horse, it is in order to know (the way to) the heavenly world. 'A cloth, an upper cloth, and gold,' this is what they spread out for the horse: thereon they quiet (slay) it, as (is done) for no other victim; and thus they separate it from the other victims.

When they quiet a victim they kill it. Whilst it is being quieted, he (the Adhvaryu) offers (three) oblations, with (Vâg. S. XXIII, 18), 'To the breath hail! to the off-breathing hail! to the through-breathing hail!' he thereby lays the vital airs into it, and thus offering is made by him with this victim as a living one.

With, 'Ambâ! Ambikâ! Ambâlikâ! there is no one to lead me,'--he leads up the (four) wives: he thereby has called upon them (to come), and, indeed, also renders them sacrificially pure.

With (Vâg. S. XXIII, 19), 'We call upon thee, the host-leader of (divine) hosts, O my true lord!' the wives walk round (the horse), and thus make amends to it for that (slaughtering): even thereby they (already) make amends to it; but, indeed, they also fan it. Thrice they walk round; for three (in number) are these worlds: by means of these worlds they fan it. Thrice again they walk round,--that amounts to six, for there are six seasons: by means of the seasons they fan it.

But, indeed, the vital airs depart from those who perform the fanning at the sacrifice. Nine times they walk round; for there are nine vital airs: vital airs they thus put into their own selves, and the vital airs do not depart from them. 'I will urge the seed-layer, urge thou the seed-layer!' (the Mahishî says);--seed, doubtless, means offspring and cattle: offspring and cattle she thus secures for herself. [Vâg. S. XXIII, 20,] 'Let us stretch our feet,' thus in order to secure union. 'In heaven ye envelop yourselves' (the Adhvaryu says),--for that is, indeed, heaven where they immolate the victim: therefore he speaks thus.--'May the vigorous male, the layer of seed, lay seed!' she says in order to secure union.