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Chart of spatial interphase
Spatial interphase, also known as interspace or an interphasic rift, is essentially a "tear in time" created in a location of space where its very fabric has weakened, and an overlap in numerous universes occurs.
Because our universe coexists with a multitude of other universes in the same physical space, these tears may allow brief periods of time in an area of their space to overlap between universes. The dimensional structure of each universe is totally dissimilar and any use of power in one universe may cause another to be disturbed.
Interphase has debilitating effects on humanoid nervous systems. Sustained exposure distorts synaptic connections in the brain, producing various symptoms: pain, tics, hallucinations, and eventually, madness. These symptoms abate if the affected individual is removed from the vicinity of interphase, but end stage sufferers become highly aggressive, and may attack others.
In 2155 in the mirror universe, the Tholians detonated a tricobalt device inside the gravity well of a dead star, generating an interphasic rift to the year 2268 in the "prime" universe. Unwilling to explore the rift themselves due to the dangers, they transmitted a distress call into the rift, hoping to lure a ship from the other side. The signal was detected by the crew of the USS Defiant, which became trapped in the rift. Unable to escape, the crew eventually succumbed to madness and murdered each other. It is possible to temporarily suppress the effects of interphase with medication that reduces synaptic activity, essentially damping the false sensory effects. Dr. McCoy developed such a drug from a derivative of the Klingon nerve gas theragan in 2268.
In a rescue attempt made by the USS Enterprise, her captain, James T. Kirk, also became temporarily trapped in the interphase. The Defiant and Kirk were briefly trapped in this universe, but continued activity by the Enterprise and several Tholian ships eventually ejected the Defiant into the mirror universe. Because the Enterprise maintained a transporter lock on Captain Kirk, he did not follow the Defiant, but remained trapped in the rift.
The Tholians had completed a web, trapping the Enterprise, and had begun towing it away; Spock took a chance and activated the ship's engines. The Enterprise briefly dropped into the rift, and was hurled a distance away, enabling it to escape the Tholian web. At this time, it was possible to retrieve Captain Kirk from the rift, barely in time, for his oxygen supply was exhausted. (ENT: "In a Mirror, Darkly" TOS: "The Tholian Web")