وَإِذَا قَالَ الْإِمَامُ: «وَلَا الضَّالِّينَ»، قَالَ: «آمِينَ»، وَيَقُولُهَا الْمُؤْتَمُّ، وَيُخْفِيهَا.
Meaning: When the imam says “nor those who are astray,” he says “Ameen,” and the follower also says it, but quietly.
Al-mu’tamm means the follower in congregational prayer. The feminine pronoun -ha in yukhfiha returns to Ameen, so the instruction concerns its quiet recitation.