Covering the knife during Bentching:
It is customary to cover the knife during the recital of Birchas Hamazon. It is not necessary to remove the knives from the table, so long as the knife is covered. [One may however choose to remove the knife rather than cover it.]
Shabbos and Yom Tov: On Shabbos and Yom Tov, the custom of many communities is not to cover the knives. The custom of the Jewish people is Torah [and one is thus to specifically leave the knives revealed].
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?
 Admur 180:6; Michaber 180:5; Shibulei Haleket 155; Rokeiach 332; Abudarham; Ketzos Hashulchan 44:5
Other customs: Some are not accustomed to cover or remove the knives during Birchas Hamazon. [Aruch Hashulchan 1870:6; Darkei Chaim Veshalom 302]
 The reason: The reason for this is because 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]. [Admur ibid; M”A 180:4; 1st reason in Beis Yosef and Taz 180:3 and M”B 180:11 in name of Rokeiach ibid]
Other reasons mentioned in Poskim: 1) Rabbeinu Simcha states that the reason why we remove the knives during Birchas Hamazon is 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. [2nd reason in Beis Yosef and Taz 180:3 and M”B 180:11] 2) Alternatively, the reason is because 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. [Mateh Moshe 304, brought in Kaf Hachaim 180:15] 3) Alternatively, the reason is because the letter end Pei is the only letter in the Alphabet that is omitted from Birchas Hamazon. This is because it stands for “Charon Af”, wrath and anger, and whoever recites Birchas Hamazon with Kavana does not receive any wrath, and his food is found before him in plenty and with honor for his entire life. We therefore cover the knife which represents wrath. [Mateh Moshe ibid; Rokeiach 336; Tashbatz 315 or 366; See Beir Heiytiv 185:1] 4) Alternatively, the reason is because in Birchas Hamazon 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. [Kaf Hachaim ibid] 5) The Arizal writes that one who is from the root of Kayin must remove the knife from the table. [Ruach Hakodesh p. 10, brought in Kaf Hachaim 180:15]
Ramifications between reasons: The practical ramifications between the reasons mentioned is regarding: a) Must one remove the knife, or does covering it suffice. b) Does it apply to all knives, or only iron; c) Does it apply on Shabbos and Yom Tov, or only during the week.
 Admur ibid; Michaber ibid; Chesed Lealafim 180:5 “One is to cover the knife or remove it”
Other opinions: According to the reason of Rabbeinu Simcha, it does not suffice to cover the knives, but the knives must be actually removed from the table. [So writes Rabbeinu Simcha brought in sources ibid “to remove the knives” and so concludes Shulchan Hatahor 180:4; Piskeiy Teshuvos 180:5] The Arizal writes that one who is from the root of Kayin must remove the knife from the table, and it does not suffice to simply cover it. [Ruach Hakodesh p. 10, brought in Kaf Hachaim 180:15] Based on this, everyone today is to remove the knife from the table and not suffice with covering it, as the sparks have become mixed and we no longer know if we are from Kayin or Hevel. [Kaf Hachaim ibid]
 Chesed Lealafim ibid
 Admur ibid; Michaber ibid; Birkeiy Yosef in name of Ramak and Rav Avraham Azulaiy, brought in Shaareiy Teshuvah 180:4
Other opinions: Some are accustomed to cover the knife even on Shabbos and Yom Tov. [Levush, brought in M”A 180:4] According to the reason of Rabbeinu Simcha, there is no difference between Shabbos and the weekday, and the knife is to be removed by all times. [Beis Yosef ibid brought in Taz ibid] Others however rule that even according to the reason of Rabbeinu Simcha, there is no need to remove the knife on Shabbos, as on Shabbos a person is not so distressed. [Taz ibid] According to the reason brought in the Arizal ibid, 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. [Kaf Hachaim 180:15]
 The reason: As only during the week is a knife required to be covered due to it being the weapon of Eisav. However, on Shabbos and Yom Tov there is no Satan and damaging forces. [Admur ibid; Toloas Yaakov brought in Elya Raba 180:7] As Shabbos hints to the world to come, in which all the severities are sweetened, and death will be abolished. [Birkeiy Yosef in name of Rav Avraham Azulaiy, brought in Shaareiy Teshuvah 180:4]
Other reasons mentioned in Poskim: As on Shabbos we do not build the Mizbeaich, and the table thus does not have any hint to the Mizbeiach. [M”A 180:4] The prohibition of waving iron on the Mizbeiach is only while it is being built, and hence since it cannot be built on Shabbos, it is therefore not relevant to require it to be covered. [Machatzis Hashekel ibid] Others however rule that even according to this reason, the knife is to be removed on Shabbos, as even on Shabbos the concept of the Mizbeaich which was built during the week is in existence. [Taz 180:3; Machatzis Hashekel 180:4]
 Shulchan Hatahor 180:4; Darkei Chaim Veshalom 302
 M”A 180:4 in name of Shulchan Araba [omitted in Admur ibid]; Daas Torah 180; Tehila Ledavid180
 P”M 180 A”A 4; Beis Avi 3:1145; Shevet Halevi 1:205; Az Nidbaru 7:2; Piskeiy Teshuvos 180:5
 The reason: As it is not possible to hew the stones of the Mizbeiach using metal knives, as the stones must be smooth. [ibid]
 Pashut, as they are not made of metal, and cannot kill, and therefore are not included in any of the reasons mentioned above for why we cover or remove the knife. [See Az Nidbaru 7:2; Piskeiy Teshuvos 180:5]
 Piskeiy Teshuvos 180:5
 Vayaan David 1:28; Piskeiy Teshuvos 180:5
 Ashel Avraham Butchach 180; See Salmas Chaim 187