The reasons for Mayim Achronim:[1]

There are three reasons recorded behind the Rabbinical institution of Mayim Achronim:

• Kedusha:[2] It is a Mitzvah due to Kedusha to sanctify oneself prior to blessing Hashem for one’s food, as after eating, the hands are dirty [i.e. Mezuhamos] from the food and are invalid for reciting a blessing.
• Sodomite salt danger of blindness:[3] In addition, the Sages made this an obligation because sodomite salt, and all salt of this property, can blind the eyes if they come in contact with the salt. Now, since Chazal[4] directed that one is to eat salt after each meal, they suspected that perhaps one will eat the sodomite property salt, and then touch his eyes and become blind. Thus, they instituted that everyone is to wash their hands at the end of a meal, even if they are being Yotzei Birchas Hamazon with another.
• Prevent murder:[5] The Talmud[6] records a story in which a woman was killed as a result of not performing Mayim Achronim. From that point and onwards the Sages made it an obligation.
• Evil spirits:[7] Upon eating, there are evil forces, known as Kelipos, which seek to receive sustenance from the food one eats. Upon washing the hands, one gives the Kelipos their due sustenance and they leave the person be. If one does not wash their hands, the Kelipos will embellish upon the person and try to cause him to sin. [It is for this reason that we recite “Zeh Cheilek Adam Rasha Meilokim” prior to washing. See Halacha 6!]

Practical ramifications:

The practical ramifications of these reasons is regarding:

1. Must one who plans on being Yotzei with another must wash [1^st reason-no, 2^nd and 3^rd reason-yes[8]]
2. Should a blessing ideally be said [1^st reason-yes, 2^nd and 3^rd reason-no[9]]
3. Should it be done even if one did not eat any salt. [1^st, 3^rd and 4^th reason-yes, 2^nd reason-no[10]]
4. Should it be done if one ate with a fork. [1^st, 2^nd reason-no, 3^rd and 4^th reason-yes[11]]
5. Should it be done after touching salt even outside of a meal? [1^st, 2^nd and 3^rd reason-no, 2^nd reason-yes[12]]

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[1] See Kaf Hachaim 181:1

[2] 1^st reason in Admur 181:1; Michaber 181:1; Brachos 53b; Rabbeinu Yona Brachos 40a; Rosh Brachos 8:6; Madanei Yom Tov on Rosh ibid

[3] 2^nd reason in Admur ibid; Michaber 181:1; Chulin 105a; Rabbeinu Yona Brachos 40a; Rosh Brachos 8:6; Rambam Brachos 6:3

[4] Brachos 40a

[5] Brought in Rif and Tanchuma Balak 15 as the reason for Mayim Achronim

[6] Yuma 83a; Chulin 106a

[7] Zohar Teruma 2:154; Shaar Hamitzvos Parshas Eikev; Kaf Hachaim 181:1; See Admur 181:2 “Due to the evil spirit which rests on the waters”

[8] Admur ibid

[9] Admur ibid

[10] See Admur 181:9

[11] See Mor Uketzia 181

[12] See Q&A below