One Minhag that is practiced in Breuers is the practice that one who leads the congregation in Selichos will also serve as chazan the evening before, and during the day when feasible.
Reb Yisroel Strauss adds that they still allow for an Avel to daven with the following qualifiers:
An Avel can daven (except 1st day, Erev Rosh Hashana and ErevYom HaKipurim) after Selichos until Yishtabach; Yahrzeit may daven starting with 2nd Ashrei. Many aveilim decline because no davar bikdusha is contained therein.
This minhag is mentioned in the Rema (581;1) from the kolbo and levush, and there is disagreement if it includes the maariv before or just the subsequent tefilos.
The reason mentioned in the Levush is the principle “המתחיל במצוה אומרים לו גמור” “One who begins with a mitzvah, we tell him “Finish!”
The application of this concept is interesrting here, since the Rema applies the minhag to the night before the selichos as well. And if the Rema was also of the opinion that the idea is to do a “complete” mitzva, then we see the application of this rule even before the mitzva has begun. I.e.- We tell the Chazan to daven maariv, so that when he does daven selichos, it will be part of a full day’s davening!
But, in truth the concept of encouraging one to do a mitzvah completely as applied here is a little different. The gemara has three references to this idea:
- “One who starts a mitzvah and doesn’t finish it, buries his wife and children”
- “The mitzvah is only counted for the one who finishes it”
- “ One who begins a mitzvah, we tell him “Finish (it)!”
The Vilna Gaon in his glosses to Shulchan Aruch, on the words of the Rema that mentions the minhag to give other tefilos to the leader of selichos references the gemara in Berachos 42b.
There the mishna says “The one who leads the Grace After MEals, is the one who makes the blessing over the incense (that is passed around thereafter)” Rashi explains that since he has begun to recite the afterblessings, he should continue with another one.
So, we have a concept, that we should give the honor of a mitzva to the person that is already doing a similar mitzvah. In other words, the idea of “Omrim lo Gemor- We tell him :Finish it” is a rule of thumb, to keep the person on stage for an encore. It is not because he would have done an incomplete mitzvah were he to just stop at that point, (in our case, to only daven selichos) but it would be more complete, and therefore he has an upper-hand in staking a claim to the rest of the davening for that day.