Real World article
(written from a Production point of view)

According to the script for Star Trek IV: The Voyage Home, the warp drive regulator was a type of sensor used aboard Federation starships. This sensor was known for generating parallex matter echoes, a condition in which the pursued ship appeared to occupy the same space as the pursuer, and science officers were expected to identify and correct this reading.

In 2286, on his memory test, this piece of technology was mentioned in an answer that Spock gave to the computer.