He who knows that the udgîtha is the pranava, and the pranava the udgîtha, rectifies from the seat of the Hotri priest any mistake committed by the Udgâtri priest in performing the udgîtha, yea, in performing the udgîtha.

In plain words: Whoever knows that the chanted hymn and Om are the same can fix any mistake the singing priest makes during the ritual. He can do this even from his own seat as the reciting priest.