While the tzadik is alive in this world, his influence is restricted by the limits of his physical existence. However, after his passing, anyone can draw from the tzadik’s true life – his belief, awe and love of God – without any physical limitations.

The tzadik’s true life is belief, awe, and love:

The holy Zohar states, “A tzadik is present in all the worlds even more after his death than during his lifetime” (Zohar 3:71b). This requires explanation. Granted, he is more present in the upper worlds since he has ascended there; but how can he be more present in this world? The answer is based on the explanation I received of the phrase used by our Sages, of blessed memory, “[the deceased] has left his life to all the living.” As it is known, the life of the tzadik is not physical, but rather a spiritual life, comprised of faith, awe, and love of God.

After his death, the tzadik’s spiritual influence is not bound by the body:

And so, when the tzadik was living on this earth, these three attributes [of faith, awe, and love of God] were contained within their “vessel” and “clothing” in a physical location, in the sense of the soul bound to his body. All of his disciples could receive only the illumination and radiance of those qualities which could shine forth from this vessel – through his holy speech and thoughts…. However, after his passing, the aspect of his soul that remains in the grave separates from his spirit residing in the Garden of Eden, which is comprised of these three attributes. Therefore, all who are close to him are able to receive a measure from his spirit in the Garden of Eden, since it is not contained in a vessel nor confined to the dimensions of a physical location. (Rabbi Schneur Zalman of Liadi, Tanya, Igeret HaKodesh 27)

Further reading: For more on visiting the graves of the righteous, see A Concise Guide to the Sages, p. 187.

The tzadik influences his disciples even after his passing. His students include not only those who actually came into contact with him during his lifetime, but even those who drew from his Torah and were strengthened through it in their own service of God.

A person can be considered a disciple of the tzadik even if he did not know about the tzadik nor come into contact with him while he was alive, but only learned of the holy books he wrote and left as a blessing after he passed away, and benefited from the illumination and radiance of his Torah, being strengthened in their service of God by those books which teach him how to serve and walk in the ways of God. Such a person is also connected with him for he believes in [the greatness of] the tzadik and receives from him the light of his Torah. (Rabbi Shalom Dovber Schneersohn, Kuntres HaHishtatĥut, p. 28)