אפוד (mentioned in this week’s parashah, see 39:5) is gematriya אמן, alluding to the mitzvah of answering amen. The efod had a belt called חשב .האפוד חשב hints at מחשבה, thoughts. חשב האפוד indicates that one should think and have kavanah when he answers amen. (Rebbe Shmelka of Nickelsburg zt'l taught this lesson.)
Before his petirah, Rebbe Shmelka of Nickelsburg said that throughout his life he was cautious that someone should answer amen to his brachos. Once, Rebbe Shmelka was traveling to a bris milah and had to say asher yatzar, but nobody was around to answer amen. What should he do? He didn’t want to make a brachah without someone answering amen. Suddenly, he saw two people. They appeared awesome. Rebbe Shmelka recited asher yatzar, they answered amen sweetly and with kavanah, and disappeared. Rebbe Shmelka asked his attendant what he saw, and he said that he didn't see anyone. Rebbe Shmelka understood that these two awesome-looking people were malachim – the malach Refael and malach Gavriel – sent down from heaven to answer amen to his brachah.
Rebbe Shmelka realized that this is alluded to in the piyut of the Yomim Nora'im, באחד והוא, when a person is alone, ישיבנו מי, who will answer amen to his brachos? The answer is, וקדוש מרום נורא ויעש Hashem will send down awesome malachim from heaven to answer amen to his brachos.
Rebbe Shmelka continued his trip to the bris milah and fell asleep on the wagon. In his dreams, it was revealed to him that he had correctly understood what had occurred. The two "people" were malach Refael and malach Gavriel. They came to answer amen to his brachah, and the hint that he found in the piyut is a true explanation.
Reb Chaim Volozhiner zt'l was also cautious that someone should answer amen to his brachos. Once, he awoke in the middle of the night to study Torah. He needed a drink, but no one was around to answer amen. Suddenly, a bachur came and asked Reb Chaim to explain a Tosfos to him. Reb Chaim asked him to answer amen to his brachos, which he did.
The next day, Reb Chaim Volozhiner thanked that bachur for being there for him to answer amen, but the bachur didn't know what Reb Chaim was referring to. Reb Chaim understood that it was Eliyahu HaNavi. He came down from heaven in the image of that bachur to help him say a proper brachah. (That bachur was accorded great honor in the yeshiva. They said that if Eliyahu haNavi appeared specifically in the image of this bachur, it shows that he is special.)
Some people have become accustomed to mumbling the brachos and to mumbling amen. It is far better when one accustoms himself to say brachos and to answer amen out loud. Chazal (Shabbos 119:) say, כוחו בכל אמן העונה כל עדן גן שערי לו פותחין, "Whoever answers amen with all his strength, the gates of Gan Eden open for him."
