Spatial interphase, otherwise known as 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. The crew of the USS Defiant, unable to escape an interphasic rift, 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 2268, the USS Defiant became lost in a spatial interphase near Tholian space. In a rescue attempt made by the USS Enterprise, her captain, James T. Kirk, also became temporarily trapped in the interphase. Although not known to the Enterprise crew, this interphase was deliberately created by mirror universe Tholians approximately one hundred years earlier; they detonated a Tricobalt warhead within the gravity well of a dead star to create the rift. The mirror Tholians regarded exploration of the rift as hazardous, so they sent a distress signal through the rift, to lure a vessel from the other side through to their universe. The rift connected the universe of 2268 to the mirror universe of 2155, and to at least one other universe (perhaps the rift itself) – one completely devoid of life. 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. (TOS: "The Tholian Web")
An Interphasic Rift can be artifically created by using an Interphasic Field Generator. Such a rift was created by the USS Rosenante in 2408 to force the ISS Rosenante back into the Mirror Universe.