The Previous Rebbe discusses the dynamic of the Rebbe/chossid relationship:
One of the novelties that Chassidus introduced was that chassidim have a Rebbe, and they live with a saying, a gesture, or a melody of the Rebbe.
When a chossid hears a saying, a gesture, or a melody of the Rebbe, at that moment his Nefesh, Ru‘ach, and Neshamah [the three levels of the soul that are vested in a body] are connected to the Rebbe.
When one reviews an aphorism that the Rebbe said, or a melody that he sang, even if one only reviews the saying superficially, this is called “a knock on the door.”
The “knock on the door” indicates that he is here, and wants to come in. One will not knock on the door if he has no desire to enter; no one would have such arrogance, G–d forbid, to knock and not wish to enter. He is not such a type of person; rather, [he is acting] superficially. This is called “knocking on the door.”
The “knock on the door” is when a person with a chassidishe education says, “Rebbe, I’m yours. I devote myself to you completely. It is only that the sly one, who is ‘clever for doing evil’ [cf. Yirmiyahu 4:22], the evil inclination, who wants to deceive me, and put me in a sack. I do not want this. I am yours; I want to be as you desire. Have pity on me, Rebbe, extract me from where I am, and bring me to stand where I should be.”
Likkutei Dibburim, Vol. 3, p. 516.
Summary: The very act of reviewing an aphorism or a melody of one’s Rebbe, even if done half-heartedly, is a “a knock on the door,” i.e., it establishes a connection between the chossid and his Rebbe and indicates that the chossid desires to connect further with his Rebbe and receive from him even more deeply, and that this is his true desire, even if he falls occasionally, and that he asks the Rebbe to pull him out of the low spiritual state in which he finds himself.