Chazak Chazak V'nitchazek
Upon completion of each of the five books of the Torah in the course of the Shabbat morning Torah reading, it is customary for everyone present to shout out loud: "chazak, chazak, v'nitchazek" which means "Let us be strong, let us be strong and let us strengthen others as well."[1] It is interesting to note that the original custom was to recite this phrase at the conclusion of each and every aliya and not merely at the conclusion of a book of the Torah, as is done today.[2] The custom in most Ashkenazi communities today is to salute the one who received an aliya with the blessing "yasher ko'ach" which can be described as an expression of "spiritual congratulations".[3] In most synagogues, the congregation rises for the last few verses of each book of the Torah in preparation for reciting the "chazak" together out loud.
There are a number of authorities who are of the opinion that "chazak, chazak, chazak" is what truly should be said upon completion of a book of the Torah.[4] This is because the gematria of these three words combined is the same as the name "Moshe". There is also a view that the proper wording is actually "chazak v'nitchazak".[5] Nevertheless, common custom is not like these views. There is also an intriguing theory that the custom to shout "chazak, chazak, v'nitchazek" upon completion of a book of the Torah is actually the result of a printer's error. It may just be that an acronym (in the form of the letters chet-zayin-kuf) for the words "chazzan v'kahal" which used to be printed in Chumashim to remind the reader and congregation to say "chazak v'nitchazek" was misunderstood and came to be printed as "chazak, chazak, v'nitchazek"
The "chazak" is recited by everyone present, to the exclusion of the one who was honored with the concluding aliya. This is because the "chazak" is in many ways, in fact, a blessing to him. Additionally, some authorities rule that the one who received the aliya should remain silent as shouting "chazak" might be considered a forbidden interruption between the Torah reading and his concluding blessing. If, however, the one who received the aliya also recited the "chazak", one need not protest and nothing need be done.[6] The one who reads the Torah only says "chazak…" after the congregation has done so.
It may just be that the wording of "chazak" is meant to parallel the blessing God gave to Yehoshua[7] or the blessing which the angel Michael gave to Daniel.[8] Although the "chazak" recited at the conclusion of a book of the Torah is viewed as a blessing to the one who received the last aliya of the book it is also deemed a congregational blessing for having reached the milestone of completing another book of the Torah together. Finally, the "chazak" is also a congratulatory blessing to those individuals who completed that book of the Torah within the context of self-study as well. Indeed, a person is required to study the weekly parsha twice along with the targum and other commentaries each week before it is read on Shabbat in the synagogue.[9]
[1] Levush 669
[2] Rema O.C. 139:11
[3] Yasher Ko'ach, or the shortened form of "shko'ach" literally mean, "May your strength be firm" first quoted in Rashi to Shabbat 87a. It is suggested that in ancient times, the Torah scroll was held upright during its reading so that those surrounding it could follow as it was being read. With this in mind, Sephardic communities made Sifrei Torah in cylindrical cases, which were self-standing. In Ashkenazi Synagogues, however, the Ba'al Koreh required a lot of strength to keep the Torah scroll upright while he was reading from it and often he was helped by those standing alongside him. Hence, a "Yasher Ko'ach" was certainly in order.
[4] Aruch Hashulchan O.C. 139:15, Elef Hamagen 669
[5] Shmuel II 10:12
[6] Piskei Teshuvot 139:17
[7] Yehoshua 1:6,7,9,18
[8] Daniel 10:19
[9] O.C. 285