When is it permissible for the one being led in prayer to begin bowing: when the Imam says "Allahu Akbar," during it, or after finishing it?

The prescribed action for the worshipper is to follow the Imam, such that he neither precedes him, nor acts simultaneously with him, nor lags behind him. Rather, he performs the action immediately after his Imam performs it. This is indicated by the Hadith: "The Imam is appointed only to be followed. So when he says 'Allahu Akbar', then say 'Allahu Akbar', and when he bows, then bow, and do not bow until he bows..." Thus, the Imam is the reference point for the actions of the prayer. The criterion is the Imam's action, not his pronouncement of 'Allahu Akbar'.