The Mirror Universe at first appears to be an Alternate universe.
The people and organizations of a Mirror Universe appear to undergo a moral inversion. People who are good become evil, people who are evil become good. The United Federation of Planets is often Mirrored as the evil Terran Empire.
However, in a Mirror Universe event, people, organizations and objects are mirrored in a fashion that doesn't make sense or follow accepted continuity and divergence theories. A ship undergoing a Mirror Universe encounter will find that it's mirror counter part has analogs to all of its crew people (At least at first). Objects, being morally neutral will often be duplicated much more directly. A Mirror Universe starship will duplicate its normal universe counterpart in great detail, despite the fact that the evil mirror universe Empire has no reason to put a bowling alley on a constitution class starship. Mirror Universe phasers will be technically identical to normal universe phasers, including a stun setting that would seem out of character for an evil empire.
Mirrored people will be earnestly unpleasant, or if antagonistic, surprisingly pleasant, and remember unlikely backgrounds as clearly and throughly as their normal universe counter part remembers his own history. Some elements however do not change for inexplicable reasons that seem to have more to do with current circumstances. The Halkan Council for example (Mirror, Mirror) was as adamantly peaceful in the Mirror universe as in the prime universe.
The phenomenon raises difficult moral, scientific and especially philosophical questions. The effect seems to be metaphorically similar to the virtual particle phenomenon in quantum physics. The mirrored starship and crew people do not represent an incursion from a stable alternate universe, but a virtual mirror universe. The circumstances "mirrored" at those currently in force, not a progression from events as they would naturally flow. The people of the mirror universe are always concurrent, even if the events of a previous mirror event would have changed things utterly, or should have.
Mirrored ships and crews most often disappear. Very often this is in the context of being "Sent home" to their "Home Universe". Ships and individuals visiting a "mirror universe" will find attempts to return home almost always succeed. This is due to the fact that the energies must balance.
In extreme cases, mirrored people and objects may become stable, "stealing" prerequisite energy from other sources. The energy required is enormous, usually only stellar phenomena throw around that kind of energy. Ion storms, stars and planets undergoing dramatically catastrophic events have generated stable mirror phenomenon.
This raises difficult philosophical questions. In effect the universe may generate a mirror John Smith. Mirror-John will usually be physically identical to Normal John Smith, but will recall a personal history that is a mirror image of Normal John's. Loved ones will become enemies, enemies will become family and friends. Now, in the rare case that Mirror-John becomes stable, what is his relationship to Normal-John? Do his memories amount to a strange stellar phenomenon, or is he remembering a real place? Although Mirror-John Smith's origins are shrouded in doubt, his current existence is real. Does Mirror John Smith share Normal John Smith's citizenship? Is Mirror-John responsible for crimes that he committed in his possible home dimension? Mirrored people often appear to be sociopathic, or at least extremely ruthless. They have the potential to commit horrible crimes. Some have attempted a cultural defense, saying that criminal acts were expected and encouraged in their home culture and therefore, a mirrored person cannot be expected to make a competent assessment of right and wrong. No such defense has succeeded, yet.
It is plain that people created from this phenomenon, those who become permanent, face a long period of adjustment and counseling to become equipped to survive in modern society. Many such people commit violent crimes and wind up imprisoned for long terms. Federation Law is deeply agnostic on the status of such people. Most who encounter stabled mirrored forms simply ignore the strange origin and treat the duplicates as citizens of the Federation who coincidentally bear precise resemblance to other people. Whether John Smith considers Mirror John Smith to be a relative, or pursues any subsequent relationship with him is currently left up to Normal John Smith and Mirror John Smith to work out for themselves.
Other variations, gender reversal, role reversals or perhaps stranger mirror phenomenon are not excluded by the current understanding of the phenomenon, but have not been recorded, yet.
The current record holder for such "Quantum Clones" is Adm Li'ira Mary O'Keefe who now has two stable duplicates. The record for unstable duplicates was more than 285,000 - which the Enterprise-D encountered on stardate 47391.2.during a quantum fissure in the space-time continuum near the Argus subspace telescope array.
The existence of this phenomenon also calls into question the entire concept of alternate universes. Do they exist or are they temporary turbulence in time and space? This question has led to fisticuffs among temporal physicists.
Garry Stahl invented this mechanism to explain the Mirror Universe. It explains why the different Mirror Universe stories are not consistent with each other, as well as explaining the internal inconsistencies in the Mirror Universe concept itself.
The idea allows for interesting story potential and conflicts.