Covering the knives before Birkas Hamazon (abridged)MBY 180:5 [Quoted from Kitzur Shulchan Aruch: 44:4 – Laws Concerning the Washing of Hands After a Meal. Translation relies heavily on the “Metsudah” edition, by Rabbi Avrohom Davis]
Ed. Here is a fascinating ‘minhag’ (custom), which I have seen practiced (although perhaps not common in all places…)
It is customary to remove all metal knives from the table before Birkas Hamazon or to cover them. Here is the primary reason for this minhag: Concerning the Mizbeiach (Altar) of the Beis HaMikdash ( Holy Temple), the Torah states (Devarim 27:5): “Do not lift up any iron upon them.” The Sages explain the reason for this prohibition as being that since metal shortens human life (i.e. in the form of weapons), it is improper for something that shortens life to be raised above that which lengthens life (i.e. the Mizbeiach). Similarly, a man’s table prolongs his days and atones for his sins, when he invites needy guests to dine with him. The power of hospitality is so great that it causes the Divine Presence to dwell in our midst! Therefore, when the meal is over, and it is time to recite Birkas Hamazon, we remove or cover the knives in order to symbolize this idea. Generally, this is not done on Shabbos and Yom Tov, because it is taught that knives (i.e. swords) represent the power of Eisav, and on Shabbos and Yom Tov these forces of evil do not prevail. Indeed, every (bona fide) ‘minhag’ (custom) in Israel has the validity of Torah.