Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.2.16

3.3.2.16

yadvevodācaṃ nyācam mimīte | imā evaitannānāvīryāḥ karoti tasmādimā nānāvīryāstasmādvā udācaṃ nyācam mimīte

And, again, as to his meting out in bending them up and down,--he thereby harnesses a virâg (to ply) thitherwards and hitherwards: going thither-wards, namely, it conveys the sacrifice to the gods, and coming hitherwards it assists men. This is why he metes out in bending (the fingers) up and down.