The Derech Moshe writes, "Teshuvah is very great, and by doing teshuvah [when one stops speaking during davening], it benefits others [for they will learn from him also to be cautious]. The sins will become merits... you and your children will merit a good, pleasant life."
The Gemara (Shabbos 119:) says, "Whoever answers amen yehei shmei rabba with all his might, his decree is ripped up... Even if he has the disgraceful sin of avodah zarah, he is forgiven." Why does amen yehei shmei rabba grant people life and annul heavenly decrees?
The following mashal will help us understand:
A king announced that he would be visiting a certain distant city on a specific date. The residents of that city prepared for the king's visit. With each passing day, the preparations and the excitement mounted. When the day finally arrived, everyone came to the city limits to greet the king, shouting loudly and joyously, 'yechi hamelech — long live the king!' They shouted this several times with all their hearts and soul. But suddenly, a barrage of stones hit the king! Everyone was stunned into silence. The police quickly sought out the culprits who threw the rocks. It was two youths who were immediately arrested and jailed. A date was set for their trial.
At court, the prosecuting attorney spoke about the severity of what these youths perpetrated, that they rebelled against the king. The defense attorney, however, said, "This wasn't a rebellion. I'd call it childishness. When everyone shouted yechi hamelech, I was standing near these boys and they were shouting yechi hamelech loudly and enthusiastically together with everyone. This proves that they are loyal to the king. That they threw stones is certainly childish behavior and shouldn’t be considered a rebellion.”
The nimshal is that sometimes people err and sin. The prosecuting malachim claim that our deeds are morred bemalchus, a rebellion against Hashem, chalilah. But if we will shout together with the congregation יהא אמן רבא שמיה it shows that we truly want to increase Hashem's honor and that our sins are accidental, influenced by the yetzer hara. This is why saying amen yehei shmei rabba annuls harsh decrees and atones for all sins, even avodah zarah.
