2. Question: Why technically if you don’t answer amen but you just had in mind and the person saying the blessing had you in mind then it’s as if you said the blessing by any blessing besides if one bentches gomel upon hearing someone being saved, where the person actually saved must answer amen to fulfill his obligation of bentching gomel?
Background:
A. The Mishna Berura (8:5:15) says that if 2 or 3 people are putting on their tallis all at the same time they should make the blessing all at the same time and if they want one can say it for everyone and the rest answer amen and even though one should anyways always answer amen to a blessing, but in this case when they want to fulfill the obligation themselves then answering amen shows they want to be apart of it, but technically even if they didn’t answer amen they fulfilled their obligation for the blessing [by just listening and having in mind.]
B. When bentching gomel for himself a person should say Blessed is Hashem…Who has bestowed every goodness upon me. The Mishna Berura (219:4) says that if someone else said Blessed is Hashem… Who has bestowed every goodness upon you, and you answer amen then it works for yourself.
Answer: Rebbe Akiva Aiger quoted in the Be’ur Halacha (219:4 “vi’ana amen”) says that since the listener cannot say “bestowed goodness upon you” then he cannot fulfill his obligation with the language of that blessing so listening is like answering would not apply and therefore he must answer amen which means he is acknowledging the subject of the blessing and not specifically how it was said.