The final concept cited by Rabbi Isaac from the Tanya is that of hashra'ah. This is a difficult term to translate. Its root–shara (to "immerse")–implies the pervasive infiltration of some higher element or force within a lower one. It is commonly used in reference to the Shechinah, God's universal Presence within the created realm. Hence we have identified it as the concept of Divine omnipresence.
The term hashra'ah is also used in the vernacular to denote "inspiration," implying an infinite force encompassing one's reality which elevates him to a transcendent plane which otherwise would be unattainable. For instance, the inspiration one derives from immersing himself regularly in the presence of a great tzadik is such that he may often become endowed with some of the tzadik's own capacities, even though he is still in essence the "lesser" individual that he was before.
The dimension of Kabbalistic thought introduced by the Ba'al Shem Tov is what allowed for a fuller appreciation of God's omnipresence within Creation. Although the concept of God's immanence within the created realm was always a central one within Kabbalah, the implications of this concept as expounded by the Ba'al Shem Tov amounted to an entirely new revelation. According to the Ba'al Shem Tov, Divine immanence implies a direct equivalence between God and all other levels of reality, as expressed by the Chassidic aphorism: "All is God and God is all." The proper understanding of this idea, especially as it differs from that of pantheism, represents the supreme insight to be attained prior to the Messianic age.
The presumption of a stratified reality, be it one which is statically hierarchic (as described by the Ramak) or dynamically interactive (as described by the Ari), is one intuited by finite minds unable to grasp the true nature of existence. Although both the system of the Ramak and the Ari play an important role in advancing our awareness of the Divine element within Creation, they are only stepping stones on the path to a fully liberated consciousness capable of seeing God within all reality and thus attesting to His absolute exclusivity of Being.
Hence the Ba'al Shem Tov touches little upon the topic of gilgul, the notion of overlapping realities giving way to the awareness of God interpenetrating all of reality in equal measure, the essence of hashra'ah. The realization that every individual being is subsumed within a higher, more infinite, collective being renders the historical consideration of gilgulei neshamot virtually irrelevant. As Jews become more conscious over time of their identification with a Divine collective being, their focus shifts from that of concern with their own individual selves to one of concern with the broader identities of community and nation. This tendency is expressed as well in the desire to reclaim the collective homeland of the Jewish people and reestablish there the rhythms of a shared existence in dialogue with the Divine.
Hence the aspect of hashra'ah introduced by the Ba'al Shem Tov represents the final dimension of Kabbalistic thought to reveal itself before the coming of Mashiach. All of Kabbalah is now to be understood in terms of this revelation, albeit in context of the conceptual progression that preceded it through the systems of the Ramak and the Ari.
More information about Kabblah you can read in "What you need to know about Kabbalah"