Delaying Melacha until after Maariv:
In addition to waiting until three small consecutive stars are visible and saying Havdala in prayer or saying Baruch Hamavdil, every meticulous person is to delay doing any [Biblical] Melacha until the congregation has recited the Seder Kedusha after Maariv. [One who is Davening in private at home is to wait until the congregation which he Davens at finishes the Seder Kedusha.] Any person who does Melacha before this time does not see a good omen from this Melacha. Even a candle should not be lit until this time, unless it is being used for a Mitzvah purpose, (in which case it may be lit after Barchu). [However today the custom is no longer to delay until Barchu is recited, and rather one may do Melacha for the sake of a Mitzvah even prior to Barchu.]
Rabbinical Melacha prior to Seder Kedusha: One may move a candle and do [any Rabbinical] Melacha of the like prior to Seder Kedusha if the time of night for the leave of Shabbos has arrived and one said Baruch Hamavdil.
Additional customs of delaying Melacha:
Delaying Melacha until after Havdala: Some Poskim write that those which are meticulous are to delay doing Biblical Melacha until after they hear Havdala over wine. Others rule that from the letter of the law one may not do any time consuming Melacha until Havdala. Thus according to them one must avoid writing, sewing and laundry until after Havdala. However one may wash dishes and sweep the floors and the like. According to Admur there is no need to suspect for these opinions.
Not to do Melacha until after Melaveh Malka: Some write since the extra soul does not leave until after Melaveh Malka, it is therefore proper to delay doing Melacha which is unconnected to food preparation until after one finishes eating Melaveh Malka. Others write one is not to do any time consuming Melacha until this meal. Based on Kabala one is to avoid even learning Torah until this Seuda.
May one smoke cigarettes before Maariv after saying Baruch Hamavdil?
It is improper to smoke cigarettes after Shabbos until the congregation has completed Maariv.
May women do Melacha prior to the conclusion of Maariv in Shul?
Yes. The above restriction against doing Biblical Melacha prior to the recital of Seder Kedusha after Maariv is only relevant to men. Women may begin doing Melacha once the proper time has arrived and have said Baruch Hamavdil.
If the electricity went out on Shabbos may it be turned on before Barchu of Maariv?
Yes. It may be lit after reciting Baruch Hamavdil Bein Kodesh Lechol.
Is a mourner to delay lighting candles for the Amud until after he says Barchu?
There is no need to do so, as long as he says Baruch Hamavdil prior to lighting the candles.
May a Shul begin setting up the accustomed video of the Rebbe after Shemoneh Esrei of Maariv but prior to Ata Kadosh being recited?
No. They are to delay doing so until after Ata Kadosh.
May one who has not yet said Hamavdil ask another Jew to do Melacha for him?
One who has not yet said Hamavdil on Motzei Shabbos, in which case he still may not perform Melacha, is nevertheless permitted to ask a Jew which has already said Hamavdil [either in Maariv or on its own] to do Melacha on his behalf. Thus one who continues the third meal into Motzei Shabbos may ask a Jew which has already said Hamavdil to do Melacha on his behalf, and benefit from it at that time, even though he will still mention Shabbos within his Birchas Hamazon.
In order to be allowed to do Melacha after Shabbos one must wait until a row of three small stars are visible, and is to then say Havdala in Davening or over wine or is to say Baruch Hamavdil Bein Kodesh Lechol. From the letter of the law once any of the above have been done he may then do Melacha. However those which are meticulous avoid doing any Biblical Melacha until the congregation has concluded Seder Kedusha in Maariv. [Furthermore there are those which avoid doing Melacha until Havdala over wine. Others avoid Melacha until after eating Melaveh Malka.]
 299/20; brought in Darkei Moshe in name of Oar Zarua, and so writes Zohar
 Lit. Baal Nefesh
 As will be explained below.
 This is based on the Zohar which is very stringent on this matter. [see M”A 299/17; Piskeiy Teshuvos 299/15] The Zohar [brought in Kaf Hachaim 293/7] that doing Melacha before Kedusha Desidra is repelling to Hashem.
 Ketzos Hashulchan 95 footnote 5. Vetzaruch Iyun if majority of the city has completed Maariv if one is still to wait on his congregation. [ibid] See Piskeiy Teshuvos 299/15 which suggests based on the above Ketzos Hashulchan that one does not need to wait for his congregation if majority of the city already finished Maariv.
 Parentheses are in original. However before Barchu it may not be lit. The source for this ruling is in the Rama 299/10; Ketzos Hashulchan 95 footnote 6 explains the reason for this ruling of the Rama is because in times of the Rama they would not say Baruch Hamavdil and hence Barchu would represent the leave of Shabbos. Thus they would delay until then. Now although according to this reason one who says Baruch Hamavdil should not be limited to wait until Barchu, nevertheless we do not desire to swerve from this custom of waiting until after Barchu. [This ruling of Admur to wait until Barchu even after saying Baruch Hamavdil is an original ruling not brought in other Poskim, and hence perhaps for this reason it is written in parentheses.]
Alternatively Piskeiy Teshuvos 299 footnote 132 explains the reason is because according to the Rama the holiness of Shabbos extends until after the recitation of Barchu as is evident from the ruling that one is to lengthen Barchu [Admur 293/4]. Thus one is to delay Melacha until after Barchu.
Alternatively Kitzur Halachos p. 121 explains the reason for waiting until Barchu is because Admur rules one is not to do Melacha in front of the congregation prior to Shabbos being let out with the saying of Barchu. Since this ruling is a novelty that has not been brought in other Poskim, therefore he writes it in parentheses. [See there in length for the basis of this explanation.]
 Ketzos Hashulchan in 95/2 completely omits the need to wait until Barchu, and in footnote 6 he explains it is no longer the custom to do so. So concludes also Piskeiy Teshuvos 299/17
Similarly today the custom of all is to remove their shoes at home when Tishe Beav falls on Motzei Shabbos, and they do not delay to remove his shoes after Barchu, despite this being the clear ruling of the Rama in 553/2.
To note that this was likewise the custom of the Rebbe to enter Shul for Maariv wearing non-leather shoes, despite the Rama’s ruling to delay until after Barchu. [See Hiskashrus 950]
 Machatzis Hashekel end of 299, ruled also in Piskeiy Teshuvos 299/15; as Rabbinical Melacha some even allow to be done during Bein Hashmashos regularly, and certainly it may be done now before Seder Kedusha.
 Toras Shabbos 299/15; Chesed Leavraham brought in Kaf Hachaim 299/59; Ben Ish Chaiy Vayeitzei 22
 The reason for this is because according to the Zohar one is not to do Melacha until after Havdala. Likewise according to the Rambam and Rosh one may never do Melacha until after Havdala over wine. Hence although this is not the ruling opinion, one who is meticulous is to suspect for it. [See however Admur 299/18 which brings that Rabbeinu Yerucham rules that even according to the Rambam/Rosh the prohibition applies only to time consuming Melacha while those Melachas which can be done without effort, such as lighting a candle, may be done before Havdala. Admur concludes that we do not rule like the Rambam/Rosh altogether.]
Who lights the Havdala candles according to this meticulous opinion? The Toras Shabbos ibid rules a woman or child is to light the candles and not an adult. The Aruch Hashulchan 299/23 however rules one may light the candle even according to the Zohar.
 In Shaareiy Tziyon 299/51 he brings the Derech Chaim which rules one may not do time consuming Melacha until after Havdala. So rules also Siddur Yaavetz in order so one not come to forget to say Havdala. Admur clearly does not suspect for this opinion, as explained in the previous footnote.
 See Piskeiy Teshuvos 299 footnote 148
 Shaareiy Teshuvah brought in Ketzos Hashulchan 100 footnote 8
 Yaavetz, brought in Ketzos Hashulchan ibid
 Peri Eitz Chaim; Mishnas Chassidim, brought in Ketzos Hashulchan ibid
 Piskeiy Teshuvos 299 footnote 125 quotes this ruling in name of Kaf Hachaim 299/65, although in truth one who looks there sees he never discussed smoking before Maariv but rather before Havdala. In any event the ruling stands true based on the ruling above.
 As it is best not to do Melacha until after Seder Dekedusha, as stated above.
 Toras Shabbos brought in Piskeiy Teshuvos 299/15
 See above in C and the footnotes there.
 263/25; 299/21
 In Maariv or on its own. This commonly occurs when one delays Davening Maariv or continues Seudas Shlishis [or Seder Niggunim] past nightfall. [ibid]
 It is even permitted for one to benefit from this Melacha before he says Hamavdil. [ibid] See Kuntrus Achron 263/8 for a lengthy discussion in why this is permitted. In short the reason is because the time before and after Shabbos which is considered still as Shabbos for oneself is called Tosefes Shabbos and one only accepts upon himself by Tosefes Shabbos to avoid doing Melacha himself, and not to avoid someone else doing Melacha for him.