http://www.meade.com/educational/etx...s/chap10b.html a short video on how to train drives.
The most simplest form, imagine a motor with an encoder on it. Then imagine the motor is connected to a 300 to 1 reducer gear box, then again that connected to a 180 tooth worm wheel.
This setup has a large amount of backlash. How ?, well if you can move the worm shaft back and forth, then imagine the motor would need to rotate, lets say 8 times to move the worm the same amount when its meshed. So then you are losing 8 rotations of the motor, and given it has a encoder on the motor shaft (On the Back), it assumes you have moved the scope by that amount. But in fact you havent. Now imagine this thing going on every time you slew or move or guide back and forth. This is why you train your drives. It basically will count how many pulses it will need before it should start with the real position co ordinate change.
I have obviously hyped up some figures and techniques, but i hope you now have a jist of it.
PEC will just compensate an already meshed gear system. Remember that you cannot reverse the R.A axis when your doing a PEC correction. it just slows the motor, and the earths rotation causes the star to move across your eyepiece. Plus you dont lose any pulses from the encoder, as like i said the gears are meshed in a forward motion (Sidereal).