A temporal vortex is a rift in the spacetime continuum that allows a vessel to travel from one point in time to another, which can be opened artificially. Their destination point in time can be precisely targeted and achieved.
In 2373, a Borg sphere generated a temporal vortex leading to 4 April, 2063 in orbit of the planet Earth, through the controlled emission of chronometric particles. In pursuit of the sphere ship, the Federation starship USS Enterprise-E was caught in a resulting temporal wake, and was thus protected from the changes in the timeline wrought by the Borg incursion into the past. Following the Borg into the 21st century, the crew of the Enterprise was later able to recreate the Borg's temporal vortex by adjusting the Enterprise's warp field to match the chronometric readings of the Borg sphere, and return to the 24th century. (Star Trek: First Contact)