Question:
If someone response with an incorrect “amen”, does his amen counts like he said amen or is it considered invalid or insignificant? Thank you
Answer:
When answering amen we are not allowed to answer it too early, (called Amen Chatufa- lit. a grabbed amen, by answering amen before the person saying the bracha finishes saying the bracha) or too late, (called Amen Yisoma- lit. an orphaned amen). A person that says an Amen Chatufa, (or Yesoma) is not considered to have answered amen to the bracha, and it doesn’t count. The Biur Halacha ( 124 D:H V’yesh) says that even if a person is unsure if his amen will be yesoma or chatufa he should better not say it at all
Sources:
Notrei Amen chap.7 ftnt. 3.