How did Rav Acha bar Yakov's prayers vanquish the serpent? How did each of
his prostrations cause one head of the serpent to fall off?
(a) The MAHARSHA cites the Gemara in Bava Kama (16a) which says that the
spine of a person turns into a serpent (Nachash) after seven years in the
grave if the person did not bow during his lifetime at the "Modim" prayer.
TOSFOS there (16b, DH v'Hu) explains the Gemara there based on the Gemara in
Berachos that says that when a person bows down at "Modim," he should
straighten himself like a snake (with his head coming up first). Hence, if
he does not bow during "Modim," then his punishment is that his spine
becomes a snake, Midah k'Neged Midah.
According to this, bowing at "Modim" is effective to vanquish the powers of
the serpent. That is why every time Rav Acha bar Yakov bowed down, he was
able to humble the serpent.
The Maharsha adds that the seven heads of the serpent corresponded to the
seven powers of Tum'ah that the primeval snake brought into the world (and
the seven curses given to Adam ha'Rishon, and the seven years after which
the spine turns into a snake).
(b) The SEFER HA'MIKNAH adds that this is also why Yakov Avinu prostrated
himself seven times before Esav when Esav approached him (Bereishis 33:3).
Esav's ministering angel is identified with the primeval serpent (Midrash
Rabah, end of Devarim), which has seven qualities, or powers ("Kochos"), of
Tum'ah. By bowing down seven times, Yakov Avinu was able to humble the seven
powers of the serpent and thereby peacefully co-exist with Esav.
(c) The Sefer ha'Miknah points out, further, that the Gematriya of the name
"Yakov" (182) is equal to seven times the Gematriya of the Holy Name of
Hashem (26 X 7). When his father, Yitzchak, gave seven blessings to Yakov,
he endowed Yakov with seven powers of Kedushah, holiness, that are
represented by his name.
But the name "Yitzchak" (208) is equal to eight times the Holy Name of
Hashem (26 X 8). When Esav cried out to his father, "Have you not one
blessing left for me," he wanted to receive the eighth power of holiness
from his father. It is this power of Kedushah that his father gave Esav that
keeps Esav, and his descendants, alive. This is alluded to in Esav's name,
the Gematriya of which (376) is equal to seven times the word "Tamei" (50 X
7)), representing the seven powers of Tum'ah, plus another 26 representing
the Kedushah of the Name of Hashem. When Yakov Avinu bowed down seven times,
his humility (which was directed towards Hashem, and is hinted at in his
name, "Yakov" -- "*Ekev Anavah* Yir'as Hashem") removed the powers of Tum'ah
from Esav so that only the Ko'ach of Kedushah remained, and they were able
to live like brothers (as the verse says in Bereishis 33:3 that his
prostrations led to the effect of "Ad Gishto Ad *Achiv*").
(d) We might add that this is the intention of the Midrash there which says
that Esav tried to bite Yakov's neck, but his neck turned as hard as stone
and, as a result, Esav's teeth broke, as the verse says, "Shinei Resha'im
Shibarta" -- "You broke the teeth of the wicked" (Tehilim 3:8). The Midrash
means that by Yakov's humbling himself to Hashem, he was able to remove the
Shen (the Gematriya of which is 350, which represents the seven powers of
Tum'ah, as stated above) from Esav. This is also the intention of the Ba'al
ha'Hagadah when he writes that we should respond to the wicked by "loosening
his teeth" ("Hakheh Es Shinav").
This is the same Shen -- which comprises the letters "Shin" and "Nun" --
which surrounds the letter "Ches" in "Nachash" (Nun-Ches-Shin) and the
letter "Tes" in "Satan" (Shin-Tes-Nun), which, when bound together, spell
"Chet" (sin). The VILNA GA'ON (in Kol Eliyahu, Sanhedrin 78a) writes that
this is what is alluded to by the words of the Gemara (ad loc.) "the poison
of the serpent stands between his teeth ("Bein Shinav")." The letters of the
word "Shen" (which represents the powers of Tum'ah, as mentioned above)
enclose the "Ches" and "Tes" (which spell "Chet") in the words "Nachash" and
"Satan." "Chet" is the true "poison" of the serpent that brings death to the
world (see Mishnah, Rosh Hashanah 29a and Gemara, Berachos 33a).
(e) We can understand the story of Rav Acha bar Yakov in a philosophical
way, on the same note.
A certain "enlightened" individual, who denied the existence of Hashem and
His Torah, was accustomed to enter the Beis Midrash at night, when Abaye was
not there, and to weaken the faith of the students with his arguments. He
would try to dissuade them from the study of Torah and enjoin them to study
instead the "seven wisdoms" of the physical world, which he considered to be
superior to all other forms of study.
When he entered into a discussion with Rav Acha bar Yakov, the Tzadik was
able to point out to him the flaws of all the physical wisdoms. Through his
Yir'as Shamayim and pure faith, which derived from his humility before his
Creator, Rav Acha bar Yakov forced the "serpent" to succumb, so that he
never returned to plague the students again. (M. Kornfeld)