BAsically what the title says. After the part, that I have sinned through my own fault , why do ppl hit their chest?
It is an expression of being sorrow, admitting our sins and asking for forgiveness.
This is a very ancient sign of sorrow that has fallen out of use in the modern day world besides in the Catholic Mass. If you read any ancient texts that involve funerals it will usually talk about the mourners beating their chests. And so as we are mourning for having offended God we beat our chests in sorrow.
In Jesus’ parable of the pharisee and the publican (Luke 18:9-14) the publican "…kept striking his breast, saying, “
‘O God, be merciful to me, the sinner!’”
For some reason the tradition arose here where they do it at the elevation of each individual species when they ring the bells. I can figure it out. Three times when the host is elevated . Three times when the cup is elevated. Not when they both are elevated together. I don’t get the connection. I saw this at my previous parish as well.
I have never seen this done at Mass before although I realize that we are supposed to do it. At least I think we’re supposed to do it.
The ringing of the bells during the Eucharistic Prayer is a tradition which is found in the Extraordinary Form of the Roman Rite (aka the Missal of Bl. John XXIII, the Tridentine Mass, and the Traditional Latin Mass). The bells are rung so that the congregation knows the consecration has taken place and to alert them to the elevation of the Host and Chalice so they can adore it. In the Extraordinary Form, the bells are rung at other times as well: a) before the Offertory Prayers, b) between the Preface of the Eucharistic Prayer and the Sanctus (Holy, Holy, Holy), c) just prior to the prayers of Consecration, and d) each time the Domine, non sum dignus (Lord, I am not worthy) is said (three times by the priest, and three times by the people).