Please Explain How A Threading Dial Works.

I've been watching some videos on threading. I saw a guy make a cut then move the carriage back set the depth of the next cut and wait for the threading dial to hit 1 and then engaged the half nuts. He did it every pass. I don't see how that guarantees the cutter will cut in the same thread as the last cut. What if the carriage was back a little farther than the previous cut? I'd like to know where to set the carriage and when to kick in the half nuts for each pass and keep cutting in the...

Please Explain How A Threading Dial Works.