When the chazan reaches "Modim" during his repetition, the congregation recites "Modim D'Rabbanan." Do members of the congregation bow here? If so, in what manner should they bow?
Shulchan Aruch (127:1): When the Shali'ach Tzibur reaches Modim, the Tzibur says Modim D'Rabanan and bows with him. They should not bow too much. Some say that one must bow also at the end of Modim D'Rabanan. It is good to be concerned for this opinion.
Ramah: Some say all of Modim D'Rabanan in one bowing. This is the custom.
Mishnah Berurah (5 and Bi'ur Halachah DH v'Yesh): People follow the Rema, but the Maharshal holds that one straightens up a little at Hashem, and bows again at the end. Chayei Adam holds that this is primary.
The Ari Zal holds that one bows only at the beginning.