The part of the liturgy you are speaking of is the prayers involved in the Holy Spirit making the bread and wine into the body and blood of Christ.
Words of Institution/Concecration:
(at this point, everyone, especially those in the sanctuary bow their heads and keep silent in reverence, save for the responses)
Priest: Take, eat: this is My Body, which is broken for you for the remission of sins.
Everyone: Amen (bell rung)
Priest: Drink of it, all of you: this is My Blood of the New Testament, which is shed for you and for many, for the remission of sins.
Everyone: Amen (bell rung)
Holy Oblation:
Priest: Remembering, therefore, this command of the Savior, and all that came to pass for our sake, the cross, the tomb, the resurrection on the third day, the ascension into heaven, the enthronement at the right hand of the Father, and the second, glorious coming, We offer to You these gifts from Your own gifts in all and for all. (bell rung)
The Epiclesis:
Priest: Again we offer unto Thee this reasonable and bloodless worship, and we ask Thee, and pray Thee, and supplicate Thee: Send down Thy Holy Spirit upon us and upon these gifts here offered.
Priest: and make this bread the Precious Body of Thy Christ,
Everyone: Amen (bell rung)
Priest: And that which is in this Cup, the Precious Blood of Thy Christ,
Everyone: Amen (bell rung)
Priest: Changing them by Thy Holy Spirit.
Everyone: Amen, Amen, Amen (bell rung 3 times; prostration made if not a Sunday)
(after this point, everyone raises their heads from being bowed)
http://orthodoxwiki.org/EpiclesisAlso, the bell is rung 12 times during the Nicene Creed at each "clause" (if that is the right word) and a few others at specific parts.
It is also rung during the Great Doxology just before the beginning of the Liturgy. (as well as during the Magnificat during Matins, and during "It is Truly Meet" after the Anaphora.)