Apart from announcing the start of Mass the bell is rung* four times during the celebration of the Mass – to tell people that something important is happening. For the same reason the bell in the tower is rung at the Consecration.
1. At the epiklesis, from the Ancient Greek ἐπίκλησις "invocation" or "calling down from on high" – when the priest holds his hands over the bread and wine. The ‘laying on of hands’ goes back to the Old Testament and is still used in the church today. Both priests and deacons are ordained by a bishop laying his hands on their heads. During the Mass it represents the priest, standing at the altar in the person of Jesus Christ, invoking the power of the Holy Spirit to change the gifts of bread and wine into the Body and Blood of Our Lord.
2. At the elevation of the consecrated host – to underline that this is now no longer bread but the Body of the Lord.
3. At the elevation of the chalice after the consecration – similarly to underline that this now no longer contains wine but the Precious Blood of Jesus.
4. When the priest takes Holy Communion and drinks from the chalice.
“If the sound is pleasant and servers are trained to use it reverently and sensibly, the bell enhances and accentuates the solemn moments of the sacred action.” (Elloitt. 113)
To ring the bell “reverently and sensibly” for 1 and 4 above quietly say “one thousand” while ringing the bell then stop. For the two elevations (2 and 3) say “one thousand, two thousand, three thousand” while ringing the bell then stop.
*Except at the Mass of the Last Supper when the bells are rung at the Gloria ad then remain silent until the Easter Vigil.