Periodic error correction (UTILITIES-PEC) counteracts worm gear errors to prevent trailing and improve photographs. As the worm turns, irregularities in its shape cause the drive to move in a wavelike pattern. The CGE applies PEC in three steps. First, as you guide on a star, it references the current worm position. Second, it records your guiding corrections over a period of 8 minutes (the rotation period of the worm is 8 minutes). Third, it plays back the errors to greatly reduce the size of guiding errors and any remaining manual corrections. It is good by itself for shorter deep-sky exposures.
Step-by step instructions on recording the PEC are in the CGE manual section on astrophotography.
PEC can be used in conjunction with the PEC Tool software for further improvements. PEC Tool is a "helper utility" (downloadable from the Celestron website). It allows you to record and playback PEC data. It also lets you backup, restore, average, and remove drift information from the playback data. The tool averages multiple runs to remove drift.
NOTE: Celestron does not supply autoguider software, which automatically counteracts guiding errors as they happen. Two freeware autoguider programs you can find on the Internet are Guide Dog and Meta Guide. Biasing the CGE mount to slightly increase error can actually help such programs. RA can be biased by either dropping or raising the counterweights to slightly imbalance the mount.