AGADAH: There is a widespread custom to open the door of one's house during the Pesach Seder when one recites the verse "Shefoch Chamascha Al ha'Goyim...," after Birkas ha'Mazon. What is the source for this custom?
(a) The Gemara discusses the problems of one who drinks in pairs ("Zugos"; see Insights to 109b). The Gemara says that if one "sees the public marketplace" between the cups of wine that he drinks, then the cups do not join together and are not considered to be a pair. Some Acharonim suggest that this is the source for the custom to open the door during the Pesach Seder. One who drinks four cups of wine on Seder night could encounter a problem of Zugos. Therefore, he opens the door in order to "see the public area" and thereby avoid the problem of pairs. (Even though the Gemara says that there is no problem of Zugos when one is in his home, the Gemara says that if one plans to go to sleep, then there is a problem of Zugos. Since most people go to sleep after they drink the four cups of wine, there is a concern of Zugos. Even though the Gemara says that there is no problem of pairs when one drinks the Arba Kosos, nevertheless the door is opened as a precautionary measure.) (HAGADAS ZECHER YEHOSEF)
(b) The BEIS HA'LEVI suggests another reason to open the door based on the opposite logic. The REMA (OC 480) writes that one opens the door to show that he is not afraid of any damaging agents on this night, for it is "Leil Shimurim," the night when everyone receives special Divine protection (109b). Why, though, is it appropriate to show this near the end of the Seder, as opposed to earlier in the night?
The answer is that at this point in the night, one is about to end the Seder with two cups of wine. One opens the door to show that he is not afraid of the potentially harmful effects of Zugos, for this night is "Leil Shimurim."
(c) The BE'ER YOSEF (Rav Yosef Salant zt'l of Yerushalayim), suggests that the Gemara implies that the houses in Yerushalayim in which the Korban Pesach were eaten were often very crowded, and people could not wait to finish their Pesach meat and leave. One is not allowed to eat some of the Pesach in one house and some in another (85b). Therefore, in order to prevent people from leaving the house before they finished eating the Pesach, the practice may have been to lock the door of the house until after Birkas ha'Mazon. Afterwards, the door was opened and the people would leave the house for the airy and roomy rooftops to recite Hallel (86a). In remembrance of this practice, the doors are opened after Birkas ha'Mazon.
QUESTION: The Gemara says that Rava would escort his Talmidim out of his home and give them four cups of wine, because he understood that anything more than two posed no problem of Zugos. The RASHBAM asks that Rava earlier implied that there is a problem of pairs with more than two, when he asserted that the only reason why the Arba Kosos do not constitute Zugos is because "a Kos Shel Berachah can join only for good, and not for bad." If not for that reason, however, the four cups would constitute Zugos.
The RASHBAM answers that although Rava was not concerned for Zugos with the Arba Kosos, he was concerned for Kishuf (as the Gemara says on 110b). Therefore, Rava explained that there is no concern for Kishuf because "a Kos Shel Berachah can join only for good."
If, however, the powers of Kishuf affect pairs in even numbers greater than two, then why did Rava escort his Talmidim out with four cups? (CHAVOS YA'IR #25 and 26; RASHASH)
In addition, the Gemara proves that since the Rabanan enacted that ten cups of wine be consumed in a Beis ha'Avel (a house of mourning), it must be that a quantity of ten does not pose a problem of Zugos. However, there should still be a problem of Kishuf with ten. How could the Rabanan make an enactment to do something which might lead to a danger of Kishuf? (CHAVOS YA'IR #26, cited by the DEVAR SHMUEL)
(a) The CHAVOS YA'IR answers that Rava gave four cups only occasionally when a Talmid would leave. Something done only on occasion is not a cause for concern about Kishuf, since it is unlikely that a sorcerer will meet up with that person at random. Similarly, the drinking of wine in a house of mourning is not a regular occurrence, and therefore there is no fear of Kishuf. The concern for Kishuf applies only to something that is an established practice for everyone, such as the Arba Kosos.
(b) The RASHASH answers the first question with a statement of RABEINU YONAH (quoted by the ROSH 10:25). One of the reasons why the Maror is dipped into Charoses is because pure Maror can be dangerous to one's health (114b). However, the Mishnah makes no mention of the need to dip Chazares in Charoses when one eats Chazares as Karpas, even though Chazares is a type of Maror. Only when one eats the Maror the second time, to fulfill the Mitzvah of Maror, does the Mishnah say that he must dip it. What is the difference between the two times that one eats Maror? If it is dangerous to eat it un-dipped, then the Rabanan should require that it be dipped both times.
Rabeinu Yonah answers that, as the Gemara says, the first time that one eats the Maror at the Seder is not to fulfill the Mitzvah, but only to motivate the children to ask questions. Even though un-dipped Maror poses a danger to one's health, people nevertheless often eat it plain and are not harmed. In addition, the practice to eat Karpas is not obligatory, and thus the Rabanan did not institute any particular way to eat it. The Mitzvah to eat Maror later in the Seder is obligatory, and thus the Rabanan decreed that one fulfill the Mitzvah in a way that poses no risk of danger to one's health.
The same reasoning might apply to the Mitzvah d'Rabanan of Arba Kosos. Kishuf is not a common problem, and people are normally not concerned with it. Rava did not protect his students from Kishuf when he escorted them out, because those four cups were not an object of a Mitzvah; had they wanted, they could have consumed only three cups. Since it involved no Mitzvah, Rava had no need to be concerned for Kishuf. In contrast, the Arba Kosos is a Mitzvah, and therefore the Rabanan had to be concerned for everyone's well-being when they instituted it. They instituted the Mitzvah only when they determined that there was no concern for Kishuf.