The Misnagdim claimed that the Chassidim reduced involved with Torah study. Well, the Misnagdim eliminated connection to the tzadick. Moreover, they eliminated interest in tzidkus at all, but that's another topic.
והנה יש עוד בחינת הארה לתלמידיו
Now, there is another kind of illumination [from the tzaddik] to his disciples.
רק שאינה מתלבשת בתוך מוחם ממש, כראשונה
However, it does not vest itself truly in their minds1 — as is the case with the first [kind of illumination] that derives from the Ruach of the tzaddik, whereby his faith, fear and love are intellectually integrated and internalized within his disciples,
רק מאירה עליהם מלמעלה
but radiates over them from above, encompassing and transcending them, for its very loftiness inhibits it from descending and being integrated within them.
והיא מעליית רוחו ונשמתו למקור חוצבו
It stems from the ascent of [the tzaddik’s] Ruach and Neshamah to the source from which it was hewn,
דהיינו, לחקל תפוחין קדישין
that is, to Chakal Tapuchin Kaddishin (lit., “the orchard of the holy apples”), i.e., to the Sefirah of Malchut in the World of Atzilut, the Divine source of souls.
ועל ידי זה נעשה שם יחוד
This [ascent] effects a union there, between the spiritually feminine Sefirah of Malchut and the spiritually masculine levels of Divine efflux that transcend it,
על ידי העלאת מיין נוקבין, מכל מעשיו ותורתו ועבודתו אשר עבד כל ימי חייו
by means of the elevation of mayin nukvin (lit., “feminine waters”; i.e., by means of the mortally-initiated spiritual arousal) constituted by all of [the tzaddik’s] actions, his Torah, and the divine service in which he engaged all the days of his life.
For, as will be explained below in Epistle 28, all of man’s lifelong labors and attainments are revealed from their former state of concealment and shine forth at the time of his passing.
ונזרעו בחקל תפוחין קדישין, אורות עליונים מאד
And in the Chakal Tapuchin Kaddishin, the soul’s source, are implanted exceedingly sublime lights,
לעומת תחתונים, אשר הם תורתו ועבודתו
corresponding to and resulting from the nether [illuminations], which are [the tzaddik’s] Torah and worship.
His divine service thus implants lofty illuminations above, which are revealed and descend below at the time of his passing.
והארת אורות עליונים אלו מאירה על כל תלמידיו, שנעשו עובדי ה׳ על ידי תורתו
The illumination of these supernal lights radiates over all his disciples who became servants of G‑d through his Torah and worship.
והארה זו, שעליהם מלמעלה, מכנסת בלבם הרהורי תשובה ומעשים טובים
And this illumination, which [radiates] over them from above, despite this transcendence is so powerful that it instills in their hearts thoughts of repentance and good deeds.
וכל המעשים טובים הנולדים מהארה זו, שמאירה מאורות הזרועים בשדה הנ״ל, נקרא גידולי גידולין
All the good deeds born of this illumination which radiates from the lights implanted in the above-mentioned orchard, are called “successive generations of offshoots.”
Since the illuminations themselves grew directly out of the lights implanted by the tzaddik, the good deeds which these illuminations in turn inspire are its offshoots of the second generation.
והארה זו היא בהעלם והסתר גדול, כמו שמש המאיר לכוכבים מתחת לארץ
This radiation is greatly hidden and concealed, just like the sun radiating to the stars from below the earth.
כדאיתא בזהר על משה רבינו, עליו השלום
Thus it is stated in the Zohar2 in reference to Moses our Master, peace be unto him,
שאחר פטירתו מתפשטת הארתו, בכל דרא ודרא, לששים רבוא נשמות
that after his passing his radiation extends in every generation to the six hundred thousand souls, all other souls being sparks of these general souls, as explained in Tanya, ch. 37,
כמו שמש המאיר מתחת לארץ, לששים רבוא כוכבים
like the sun which radiates to the six hundred thousand stars from below the earth.
As with Moses, so too with the tzaddikim who are his successors: by means of his Torah and spiritual service, every tzaddik illumines successive generations of offshoots — his direct disciples and in turn their disciples, and so on — when they follow his teachings in matters of Torah and spiritual service.
Addendum
The Alter Rebbe spoke above of those who benefit from the gifts that a tzaddik continues to bequeath after his passing, from his Ruach that remained in the Lower Garden of Eden. There he does not say “all his disciples,” but only “among his disciples.” Moreover, he adds that the extent of the spiritual bequest received by each chassid varies “according to the level of his bond and closeness to [the tzaddik], during his lifetime and after his passing, out of an abounding love.”
This is so because there the Alter Rebbe is speaking of their receiving from him the sublime spiritual attributes of faith, awe and love. Hence, since (as the Alter Rebbe concludes there) “anything spiritual is elicited only by means of an abounding love,” it follows that a bequest of such stature is drawn down only to those disciples whose bond to him was particularly close.
Here, however, at the close of the epistle, the Alter Rebbe speaks of the degree of illumination which radiates “over them from above,” encompassing and transcending them. This degree, which “instills in their hearts thoughts of repentance and good deeds,” radiates “over all his disciples,” upon all those who may be considered his disciples because they “became servants of G‑d through his Torah and worship.”
It could be argued that the former kind of benefaction, that which is dependent upon being bound with great love, refers specifically to those who were the tzaddik’s disciples during his lifetime; it is they who are close to him both “during his lifetime and after his passing.” By contrast, the kind of benefaction which is transcendent, as opposed to integrated within them, also applies to those who became the tzaddik’s disciples after his passing, in the sense that they drew closer to G‑d as a result of his teachings. These disciples are the successive generations of his offshoots in the same way that the radiation of Moshe Rabbeinu continues after his passing to illuminate generation after generation.
