From Barry Armstead's article in the "projects and articles" section of the site:
"Typically, you would have one telescope on the equatorial mount, taking a long exposure while the mount is tracking. Piggybacking on top of that, would be a second, less powerful telescope, with a guiding camera in it. This guiding camera sends corrections to the mount, based on a star that you have designated it to lock on to. All mounts have some degree of mechanical error in them, but a guiding camera and guiding program like
PHD, takes the pain out of it and keeps it all on track."