From "Star Trek: The Tholian Web (#3.9)"
I don't understand what's so special about this region of space.
Picture it this way, Mr. Chekov: we exist in a universe which co-exists with a multitude of others in the same physical space. At certain brief periods of time, an area of their space overlaps an area of ours. That is the time of interphase, during which we can connect with the Defiant's universe.
Mr. Spock? We WILL be able to retrieve the Captain at that time... won't we?
Yes. However, the dimensional structure of each universe is totally dissimilar. Any use of power disturbs it. If we are not extremely careful, we shall lose the Captain, and become trapped ourselves.