With long exposure noise reduction enabled, when the shutter closes the sensor is activated again for the same length of time to take a black frame (the shutter being closed so it is not exposed to any light). As this will still be affected by things like bleeding and hot pixels it effectively creates a map that can be then used to tidy up the original shot.
As it effectively doubles the time it takes to take a shot, a 30 second exposure takes 60 seconds, so you need to decide whether time or highest possible quality is most important to you.
Michael.