רַבִּי חֲנִינָא בֶּן חֲכִינַאי אוֹמֵר,הַנֵּעוֹר בַּלַּיִלָה וְהַמְהַלֵּךְ בַּדֶּרֶךְ יְחִידִי וְהַמְפַנֶּה לִבּוֹ לְבַטָּלָה,הֲרֵי זֶה מִתְחַיֵּב בְּנַפְשׁוֹ:
Rabbi Chanina ben Chakhinai said: One who stays awake at night, and who walks the road alone, and who turns his heart to empty matters—behold, he becomes liable for his soul.
Rebbe Nachman: He who stays awake at night—That is, one stays awake at night, secludes himself, and expresses his thoughts and feelings to his Maker. [This practice is called hitbodedut.]
And he who walks the road alone—One specifically goes on an isolated path, in a place that people do not frequent; for the ideal hitbodedut is at night and on an isolated path, both of which are conducive to attaining nullification of ego (bittul).
And he who turns his heart to empty matters (l’batalah)—In other words, one empties the heart of all mundane concerns l’batalah—for the sake of bittul (nullification of ego). Then one’s soul will be subsumed within the Mechuyav HaMetziyut [Imperative or Absolute Reality of God’s Existence].
Behold, he becomes mit’chayev (liable) for his soul—The entire world, together with his soul, is incorporated into the Mechuyav HaMetziyut.
Through hitbodedut one attains nullification of ego, at which point his soul becomes reunited back into the Absolute Reality, along with the entire phenomenal world that was created as the necessary arena for his free will. (Likutey Moharan I, 52, abridged)
Reb Noson: The most basic method of coping with all forms of suffering—whether those experienced by the Jewish people collectively, or those experienced by each individual, may the Merciful One spare us—is nullification to the Ultimate Reality. That is, one should close his eyes and completely nullify all sensory awareness until the very sense of self dissolves into the Infinite Light.
Anyone can do this, as I heard from the Rebbe’s holy mouth. Even though one may not be able to attain nullification of the ego to perfection like the great tzaddikim, nevertheless anyone can do so for a limited time if he is truly determined. (Likutey Halakhot, Netilat Yadayim Shacharit 4:4)