Covering the Knife During Bircas HaMazon

ה וּבָנִיתָ שָם מִזְבֵּחַ, לַ ה' אֱלֹקיךָ: מִזְבַח אֲבָנִים, לֹא - תָנִיף עֲלֵּיהֶם בַרְ זֶל.
27,5 You should build there an Altar to G-d, your G-d (by making these stones) into an Altar of stones; you should not use any iron (tool) on them.

Opinions for the Reason of this Din:

• Eliyahu Rabbah: Iron is connected to the power of Esau; thus, we do not use such power on the Mizbeach or on our table. This applies during the week. However, on Shabbos, such power has no affect and thus, we are not concerned.
• Magen Avrohom: a table is similar to a Mizbeiach, and regarding the Mizbeiach the verse states “Don’t wave iron over them”, as iron shortens the days of man, while the Mizbeiach lengthens the days of man, and it is improper that one wave the “shortening of life” over the “prolonging of life”. The same applies to one’s table which also lengthens the days of man and atones for his sins upon him hosting guests, as how great is the power of dining [together with others] that it causes the Shechina to rest [upon one’s home]. But we do not build or alter the Mizbeach on Shabbos, so we di not need to cover the knives on our tables on Shabbos.
• Bais Yosef (1) from the Rokeach: Since the main function of Iron (sword and other such instruments) leads to the shortening of life. Whereas, the Mizbeach through means of atonement, offers lengthening o life. These opposing positions do not mix. So, we cover the knife during the Bircas haMazon.
• Baid Yose (2) from Rabbenu Simchah: due to a certain tragedy that occurred after the destruction: A certain man was reciting Birchas Hamazon and upon reaching the blessing of Boneh Yerushalayim and remembering the destruction he took a knife from the table and stabbed himself. there is no difference between Shabbos and the weekday, and the knife is to be removed by all times
• Match Moshe: the blessing of Eisev is to live by the sword, and in Birchas Hamazon we mention the blessing of Yaakov of Tal Hashamayim and Shamneiy Haartez. We therefore cover the knife of Esev while reciting the blessings of Yaakov.
• Kaf haChaim: we request that Moshiach arrive, and in the future the sword will no longer be used, and we hence cover the knife to hint to this prediction.

The Arizal writes that one who is from the root of Kayin must remove the knife from the table. One is to remove the knives from the table even on Shabbos and Yom Tov, as the souls root in Kayin does not change on Shabbos

Q&A

• Must one cover knives that are not made of iron, but from other metals, such as gold or silver?
  Some Poskim rule one is only required to cover a knife which is made of iron, and not of other material, such as gold, silver, copper or plastic. Other Poskim, however, rule that all forms of metal knives are to be covered Practically, the custom is to cover all metal knives.
  Plastic knives: Plastic knives do not need to be covered.
• Must one cover the entire knife, or does it suffice to cover just the blade?
  Some write the entire knife is to be covered, and not just the metal blade.
• Must one cover all the knives on the table, and what is one to if there are other people still using their knives?
  Some write one is not required to cover all the knives of the table, but simply the knives that are within his reach. Certainly, one is not required to cover the knives that other people are still using.
• Must one cover the knife when reciting Meiyn Shalosh, or Borei Nefashos?
  No.