Making a mark on the Challah before Hamotzi on Shabbos:
The Mitzvah of having a whole loaf by Hamotzi: It is a Mitzvah Min Hamuvchar [even during the week] for the blessing of Hamotzi to be recited over a whole loaf of bread, and not to first cut it and only then say the blessing. Furthermore, on Shabbos it is an actual obligation to say Hamotzi over a whole loaf of bread.
Slightly cutting the bread before Hamotzi: [During the week], it is proper to slightly cut the loaf with a knife in a way that it still retains its whole status, prior to saying the blessing of Hamotzi. It is to be cut in the area that one plans to cut the first piece from in order to eat after Hamotzi. However, one does not separate the piece from the loaf until after the entire blessing is completed. We will now discuss the law regarding Shabbos:
- The law:
[Unlike the law stated above regarding the weekdays], on Shabbos and Yom Tov, one should not cut the loaf of bread at all until after the [entire] blessing of Hamotzi is recited. [However, some Poskim rule that one should nonetheless make a mark on the bread. It is made in the area that one plans on cutting the bread. Other Poskim, however, rule that even a mark is not to be made prior to Hamotzi. Practically, so is the Chabad custom to make a mark in the bread prior to the blessing of Hamotzi, and to then begin cutting the bread in approximation to that area, although we are careful not to actually cut it.]
On Shabbos and Yom Tov, the custom is to make a mark on the loaf of bread prior to beginning the blessing of Hamotzi, and to then begin cutting it from that approximate area.
Must one make an actual visible mark and scratch on the bread?
Some Poskim write that the mark must be visible. [Furthermore, some write that the mark should be slightly deep, in order to avoid the question of the Melacha of Koseiv.] Other Poskim, however, rule that there is no need to make an actual recognizable scratch. Practically, the Chabad custom is seemingly to make sure to make a scratch on the bread, but to not actually cut through it.
Must one cut the bread in the exact area that he made the mark?
No. It suffices to simply cut it in the general area if the marked area is not immediately visible.
If one cannot find where he made the mark, should he take time to search for it prior to cutting the bread?
No. If he cannot find the mark right after the blessing then he is not to delay at all to look for it, and is rather to begin cutting the bread from any area.
Is one to make a mark on both loaves of bread or only one?
The mark is only to be made on the loaf that one plans on cutting first after the blessing of Hamotzi.
 See Admur 167:3
 Admur ibid and 168:1; Seder Birchas Hanhenin 10:1; Luach Birchas Hanehnin 3:1; Michaber 168:1; Tur 168:1; Rambam Brachos 7:4; Rebbe Yochanon in Brachos 39b; Tosafus Brachos ibid; Rosh 6 Brachos 21
 Admur ibid, 168:3, and 274:2, 475:3; Rama 274:1; Tosafus Brachos 39b
 Admur ibid; Michaber 167:1; Taz 167:3; Mordechai Remez 128; Talmidei Rabbeinu Yonah Brachos ibid; Rosh Brachos 6:19; Tosafus Brachos 39b
 How much should it be cut? The loaf is to be only slightly cut in a way that when one lifts the loaf from the partially cut area the entire loaf raises up and does not fall. If, however, when one holds onto the loaf from the cut area the rest of the loaf falls off, then it is no longer considered whole, and is like a cut piece of bread for all purposes, and therefore, one should not cut more than the above amount. [Admur ibid; Michaber ibid; Tur ibid; Rosh ibid; Hagahos Maimanis Brachos 7 Os Vav in name of Mahram Merothenberg] See Machatzis Hashekel 274:1 that one is to cut the main part of the bread prior to the blessing.
 The reason for slightly cutting: [Although above it was ruled that one should recite the blessing of Hamotzi while the loaf is still whole, and not first cut it, nonetheless] one should partially cut the loaf in the area that he plans on first eating in order so he lessens the delay between his blessing of Hamotzi and the eating of the bread [as through partially cutting it before the blessing, one diminishes the amount of cutting he needs to do afterwards, and already knows the area that he plans on cutting]. [Admur ibid; Machatzis Hashekel 274:1]
The reason the loaf does not lose its whole status: Now, the loaf is not considered to lose its whole status due to this slight cutting, so long as it was not cut to the point that when one holds the area of the loaf that was slightly cut the rest of the loaf does not lift with it, and rather when one lifts the loaf from the partially cut area the entire loaf raises up and does not fall. If, however, when one holds onto the loaf from the cut area the rest of the loaf falls off, then it is no longer considered whole, and is like a cut piece of bread for all purposes. This applies even if when one holds onto the loaf from the non-cut area, then the area with the cut lifts together with it. [Admur ibid; Michaber ibid; Tur ibid; Rosh ibid; Hagahos Maimanis Brachos 7 Os Vav in name of Mahram Merothenberg]
 Admur 167:3; Rama 167:1; Mordechai Remez 128; Tosafus Brachos 39b
 The law Bedieved if the loaf was cut: Although on Shabbos and Yom Tov the loaf should be cut at all prior to the blessing, nonetheless if one forgot and did so on Shabbos like he does during the week, [it is still considered whole and] he does not lose out its Lechem Mishneh status being that so long as when he holds onto the cut area of the loaf the rest of the loaf lifts together with it, it is considered whole regarding all matters. [Admur ibid; Rama ibid; M”A 167:5; Mordechai Eiruvin 7 Remez 525; Agudah Chulin 150 in name of Mahram Merothenberg and Shiltei Giborim]
 The reason: As on Shabbos it is an absolute requirement to have whole Lechem Mishneh, and if one were to slightly cut it, we suspect that perhaps his hand may slip and ended cutting more than the above-mentioned slight amount and end up causing the loaf to lose its whole status and is thus found that he has blessed over a slice of bread. [Admur ibid; Rama ibid; Beis Yosef 167:1; Perisha 167:2; Tosafus ibid; Mordechai ibid] See Machatzis Hashekel 274:1 that one is not to cut it at all on Shabbos in order so the blessing is said over a complete Shaleim; See Rama ibid as explained in Perisha ibid who implies this as well, although the Perisha negates this
The reason we do not suspect for delay: [Although due to not cutting the bread even slightly prior to the blessing we increase in the delay between the blessing and eating, nonetheless], there is no Halachic significance to this delay of the cutting that will take place between the blessing and the eating, as a mere delay without speaking is not considered a delay. During the week, we only do so in order to fulfill the best possible option [which is to retain a whole loaf and diminish as much as possible between the blessing and eating, even though this delay of cutting is not really considered an interval]. [Admur ibid; Taz 167:3; See Admur 206:3; 166; 190:8]
 M”A 274:1 in name of Bach 167:1 in name of Rashal “The Medakdikin are accustomed to use the knife to make a mark on the loaf prior to the blessing”; Kitzur SHU”A 77:17; Mateh Efraim 625:49 and Ketzei Hamateh 81; M”B 274:5; Ketzos Hashulchan 82:2; Toras Chaim Sanhedrin 1092b based on Kabbalah; Piskeiy Teshuvos 274:4; mitted from Admur ibid; See Kaf Hachaim 274:11 and Ketzos Hashulchan 82:2 footnote 8
 The reason: As on Shabbos one cannot even partially begin cutting the bread prior to the blessing and hence we make a mark. [P”M 274 A”A 1; Levushei Serud on M”A 274:1; Kaf Hachaim ibid] This is done so that he already knows from which area to begin cutting and not need to search and decide where to begin the cutting after the blessing is said and hence diminish as much as possible between the eating and blessing. [Machatzis Hashekel 274:1; Kitzur SHU”A ibid; Kaf Hachaim ibid]
 Kitzur SHU”A ibid; See Machatzis Hashekel ibid
 Implication of Admur 167:3 and 274 who completely omits this ruling of the M”A and Bach [Ketzos Hashulchan 82:2 footnote 8]; Tzelach Brachos 39b; Beis Menucha 174:18; Kaf Hachaim 274:11 concludes “Shev Veal Taaseh Adif”
 Hayom Yom 15th Adar Rishon; Sefer Haminhagim p. 28 [Hebrew] p. 57 [English]; Ketzos Hashulchan 82:2 that so is the custom and footnote 8 that so was the evident custom of his grandfather and the Tzemach Tzedek; This is despite the fact that it was omitted from Admur ibid
 Ketzos Hashulchan ibid
 Sefer Haminhagim ibid
 Machatzis Hashekel on M”A ibid “Unlike the custom of the masses who circle the knife around the bread, but do not make any mark, and there is no visible mark, as what have they benefited by doing so, as there is no reason for this.”
 The reason As the entire purpose is so that he begin cutting from that area and diminish in delay as much as possible, and if there is no mark then the above cannot be fulfilled, and hence the passing of the knife over the loaf prior to the blessing has no meaning. [Machatzis Hashekel ibid]
 See Piskeiy Teshuvos 274 footnote 27 that so do the Medakdikin. However, from the letter of the law, there is no writing prohibition of Sirtut relevant to food. [See Shraga Hameir 3:15; SSH”K 11:15]
 Ketzos Hashulchan 82 footnote 8
 The reason: As the entire purpose of the mark is simply to decide before the blessing the general area from which one will begin the cutting, whether it be from the middle of the bread or the side or corner. [Ketzos Hashulchan ibid]
 As stated in Sefer Haminhagim ibid, and so is evident from the story with the Tzemach Tzedek
 Ketzos Hashulchan 82 footnote 8
 Ketzos Hashulchan 82 footnote 8 that so he heard from his grandfather Harav Moshe Na”ah, that the Tzemach Tzedek poked fun at those who would take time to search for the mark to begin the cutting
 The reason: As this is the entire purpose of the mark, so he doesn’t have to make a delay, and hence one who delays the cutting in order to search for the mark, is doing the exact opposite of its purpose.
 Pashut! Seemingly, the word “loaves” in plural in the English Sefer Haminhagim is a translator error.