Yissurim (suffering): elevating sparks trapped in impurity

Tzemach Tzedek, Derech Mitzvosecha, Biur Achrei p. 192

The advantage of yissurim (suffering) is that it is capable of refining the three completly impure klipos...because the yissurim come from the "other side" [of evil] thus through them one can refine the good from them.[...] When a man sins, he draws life-force of kedusha into the three impure klipos, and there is no way to elevate these sparks except through yissurim -- when the klipa is given authority over the man in by this the sparks are extracted. In Egypt the miracle was that the sparks were extracted from Above, without the need to endure yissurim under the hands of klipa.

[...]A person is not permitted to act with self-sacrifice on condition that a miracle will be performed from him, because if he sets this condition then no miracle will be performed. The reason for this is that it is impossible to refine [the sparks in the impure klipos] except by allowing the "other side" to have authority over him. Thus, when he sacrifices himself on condition of a miracle this means that he doesn't agree to endure any yissurim from the klipa at all, but when he acts with actual self-sacrifice this indicates that he truly agrees to endure yissurim from the klipa and thus it can happen that a miracle will be performed for him: to break the klipa [and elevate the spark] from Above after only a brief period of submitting to its authority.