Rebbe Nachman explains that when a soul comes into this world there is only one gate through which that soul can still ascend and unite with HaShem, the gate of emunah. Sometimes, that gate is temporarily closed (on a personal level because of some damage one has done, and a national level because of the culture of sinning that has been reinforced and strengthened) but that is only for a short while. In really extreme circumstances permission is given from on high to shut the gate permanently. In such times someone with true fear of heaven (to my understanding a Tzaddik) puts themselves in the mouth of the gate, and because of their purity, the gate cannot be closed. In this manner, the Tzaddik allows the gate of emunah to remain open, even when Bnei Yisrael doesn't deserve it. The catch, and here is the true mesirut nefesh, is that the Tzaddik can't leave the mouth of the gate or else it would be closed and so, his soul cannot ascend to unite with HaShem.
To me this echoed the words of Mosheh Rabbeinu in the end of sefer devarim, "I can no longer come in and go out." (לא אוכל עוד לבא ולצאת) (Devarim 31:2)
ps. please correct me if I've understood Rebbe Nachman's intent here. (this basically goes for any other Torah as well that I may have misunderstood)