Again, the following is based on a teaching of Rabbeinu Yehudah ben Yakar, one of the teachers of Ramban (Nachmanides). The focus is the bowing in Modim, the penultimate blessing of Shemonah Esrei. Our Sages were very strong in their emphasizing the importance of bowing in this blessing. They said that the spine of one who fails to bow in Modim will turn into a snake. Here are two reasons offered for this.
Tosfot (Bava Kama 16b) write that since we are to rise from our bow in a smooth, snaky motion (head first, then the body; Berakhot 12b), one who did not do so, will have his spine “serpentized.”
A second reason, offered by Maharsha, is that one’s failure to humble himself to God is Serpent-like behavior which is “rewarded” in kind.
As to the bowing itself, the opening bow of Modim follows our explicit mention of the future Beit HaMikdash (“restore the service to Your Innermost Sanctuary” and “Who returns His Shekhinah to Zion”). We thank God for this. Even though thanking is not bowing, thanking is accompanied by bowing, as in (Psalms 138:2), “I will bow towards Your holy chamber and give thanks to Your Name.” We bow here to invoke the bowing that precipitated the first Temples, so that the third one may soon be built (speedily in our lifetime – Amen!).
The closing bow follows our thanking God for “our lives that have been placed in Your hands and our souls that have been entrusted to You”—a reference to techiyat hameitim (the Resurrection). If we have lost the privilege to live or had our lives been entrusted to someone other than God, we would not be here. (This is related to the verse [Psalms 31:6], “In Your hand I entrust my spirit; redeem me Lord, God of Truth!” This is a plea concerning the Resurrection when God will redeem souls and bodies from their prison.)
This is why we say “and all of the living will thank You, forevermore” instead of just “the living”—“all” includes those who should no longer be alive. So we bow, for in that merit the dead shall live again.
Thus in the blessing that emphasizes the future Temple and Resurrection, our Sages stressed that we “bow like a snake” so that we not seem to be like those who don’t believe in these things.