Basically, "Pre" means that the effect is fed BEFORE the fader so any amplitude change of the dry signal will not change the wet sound. Therefore, "Post" puts the bus AFTER the fader and any level changes will affect both the dry and wet signals. Most of the time, the post setting is used. However, if you wanted to have a sound that decreased it's dry signal over time, but the effect stayed the same, you would set it to pre. For example, if you had a long reverb on the aux bus set to "Pre" and you brought a track's fader down over time, it would have the effect of that sound moving farther away from the listener with increasing reverb but less volume. Dig?
