The following is based on a talk by the Rebbe, in which he describes the continuing energy flowing from a tzaddik -- even after his soul has ascended on high, and how we can tap into it, physically.
Rabbi Shalom DovBer, fifth Lubavitcher Rebbe, famously said before his passing: "I am going to heaven, but my writings I am leaving to you."
"Tzaddikim resemble their Creator." Just as G-d embedded Himself within His Torah, tzaddikim likewise "embed" themselves within the words of Torah that they teach.
This, thus, is the meaning of Rabbi Shalom DovBer's statement, by studying "his writings" (his Torah) in a manner that leads to action, to the extent that his teachings become disseminated, one becomes bound to Rabbi Shalom DovBer himself, as he is in his state of ascent in Heaven.
The Rebbe applied the same principal to the sixth Rebbe, Rabbi Yosef Yitzchak, who also left us his writings, books, and possessions through which we can physically connect to him.
By binding oneself to the leader of the generation--by connecting to his writings, books, and so on, in which he embedded himself--all those who walk in his ways receive from the essential, eternal existence of the tzaddik's holiness.
This is primarily accomplished by studying his writings and teachings, and translating them into action; educating oneself as well as by disseminating the writings and teachings to others.
We need to attach ourselves to the Jewish leaders of each respective generation, to our Rebbe. This ensures us that we will receive from the leader's "eternity."
--Staying the Course, P. 126-130