The Rebbe notes that the reason the Alter Rebbe now goes on to say "There should also be etc." is that until now it has been explained how a Jew generates the fear of heaven through intellectual contemplation. The degree of fear he arouses will correspond exactly to the extent of his contemplation; the deeper the contemplation, the greater his fear. It also depends on how much each individual is governed by his intellect. Furthermore, it is too much to expect that all people constantly achieve a state of intellectual awareness — yet all people are obliged to stand in constant fear of heaven. The Alter Rebbe therefore now goes on to elaborate on a frame of mind which can and must exist constantly — "acceptance of the yoke of the Kingdom of Heaven." This is not attained through contemplation. Rather it comes as a result of faith alone — and this state can exist constantly in all individuals.
וגם להיות לזכרון תמיד לשון חז״ל: קבלת עול מלכות שמים, שהוא כענין: שום תשים עליך מלך
There should also be a constant remembrance (it is constant because it does not depend on prior contemplation, but rather on pure faith) of the dictum of the Sages, of blessed memory, "acceptance of the yoke of the Kingdom of Heaven," which parallels the injunction,1 "You shall appoint a king (i.e., G‑d) over you,"
כמו שכתוב במקום אחר וכו׳
as has been explained elsewhere, and so on.
This is also what the Alter Rebbe says earlier in Tanya (beginning of ch. 41): "Even though after all this [meditation] no fear or dread descends upon him in a manifest manner in his heart," still he should accept upon himself G‑d as his king, and accept upon himself the yoke of the heavenly Kingdom. As the Alter Rebbe explains there, this attribute is found within every Jew in a sincere manner, because of the nature of Jewish souls not to rebel against G‑d, the King of kings. This level of fear can therefore always be present.
כי הקב״ה מניח את העליונים והתחתונים ומייחד מלכותו עלינו וכו׳, ואנחנו מקבלים וכו׳
For G‑d, blessed be He, forgoes the creatures of the higher and lower worlds, i.e., they are not the ultimate intent of creation, and uniquely bestows His kingdom upon us, ...and we accept [the heavenly yoke].
וזהו ענין ההשתחוואות שבתפלת שמונה עשרה, אחר קבלת עול מלכות שמים בדבור בקריאת שמע
And this is the significance of the obeisances in the prayer of the Eighteen Benedictions, following the verbal acceptance of the yoke of the Kingdom of Heaven in the Reading of Shema, when we say, "...the L‑rd is our G‑d, the L‑rd is one," and so on,
לחזור ולקבל בפועל ממש במעשה וכו׳, כמו שכתוב במקום אחר
whereby one accepts it once again in actual deed, and so on (for by bowing in the course of the prayer of Shemoneh Esreh one shows one's acceptance in actual deed of one's self-nullification to G‑d), as is explained elsewhere.
