7. Hefsek-Approximating the washing to the Bentching:[1]

One must approximate the washing of Mayim Achrnim to Birchas Hamazon as much as possible.[2] [Doing so helps protect one’s meal from persecution of the Satan.[3]] The Sages estimated this to be the amount of time it takes for another four people to wash Mayim Achronim, that one may delay up until this amount of time.[4] Nonetheless, [even this amount of delay is only allowed if necessary] it is [however] forbidden to make any unnecessary interval at all between the two, [even for a mere moment].[5] This applies even if one remains silent during the interval.[6] [Nonetheless, when reciting Birchas Hamazon with a Zimun, one is not to begin until the Mizamein is ready, even if one will have to wait for more than the above amount of time.[7]]

Pouring the Kos Shel Bracha:[8] Due to the above necessity to approximate the washing to the Betching as much as possible, the wine for Kos Shel Bracha is poured prior to performing Mayim Achronim.

Speaking:[9] It is forbidden to talk once Mayim Achronim has been performed. This applies even to words of Torah, and certainly to other matters. One may not even say a blessing.[10] One is not to speak at all, even one word.[11] Nonetheless, one may recite the verse “Vayidaber Eilay Zeh Hashulchan Asher Lifneiy Hashem” after the washing.[12] However, Al Naharos Bavel or Shir Hamalos is to be recited before the washing.[13]

If one made an interval or desires to do one, after Mayim Achronim:[14] If one talked or made an interval after performing Mayim Achronim, the washing is to be repeated. Accordingly, if one washed Mayim Achronim, and something came up [i.e. bathroom, phone call, etc] and he now desires to talk or make an interval between the washing and Birchas Hamazon, then he may do so, so long as he plans to wash again prior to Birchas Hamazon.[15]

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[1] Admur 181:6; 179:5; Ketzos Hashulchan 43:1

[2] Admur ibid; Brachos 42a; Mishneh 51b and Gemara 52b

[3] Birkeiy Yozef 181:3 in name of Chesed Leavraham  and Yerushalmi Brachos 1:1

[4] Admur ibid regarding the leader of the Zimun that it is not considered a Hefsek; M”A 181:5; Taz 181:3; Rashba Brachos 46b

[5] Admur ibid; M”A 181:5; Taz 181:3; Rashba Brachos 46b

[6] So is evident from Admur ibid, that even a silent interval is an interval that is forbidden.

[7] Kaf Hachaim 181:21

[8] Admur ibid; M”A 181:1; Rabbeinu Yerucham 16:7; Based on Mishneh 51b and Gemara 52b

[9] Admur ibid; Bach 181; Elya Raba 181:9 in name of Arizal in Shaar Hamitzvos Eikev

[10] Admur 179:5

[11] Admur in Seder Netilas Yadayim 5 “ Not to make a Hefsek at all at all” and 6 “One must beware not to talk in between even a light talk”; regarding washing for bread; Ketzos Hashulchan 43 footnote 3; Elya Raba; Magen Giborim; M”B 165; Shaar Hatizyon 165:13; Piskeiy Teshuvos 181:14

Ruling of Admur in Shulchan Aruch: In the Shulchan Aruch Admur rules that there is no worry against speaking 2-3 words in between and doing so is not considered a Hefsek, unless one has already raised up the Kos Shel Bracha. [Admur 181:6 and 166:1; M”A 165]

[12] Siddur Admur; Kaf Hachaim 181:20; Piskeiy Teshuvos 181:14

[13] Siddur Admur; Piskeiy Teshuvos ibid

[14] See Admur 179:5; Ketzos Hashulchan 43:1; Ashel Avraham Butchach 181; Piskeiy Teshuvos 181:14

[15] Admur ibid

The reason: As if he plans to wash again, then the first washing does not have the status of Mayim Achronim, and does not carry its requirement of approximation. [Admur ibid]