Larry; it's when using the 'RC system' or 'Conti system' and the cue ball will pass the 'Vertical Center-line' of the table more than once after 3 cushions.Any shot more than 3 cushions, 4.5, 6...!
The 'Base' starting point calculation for the CB position is "45" instead of "50" as with any 3 cushion shot.
The CB will shorten it's angle as it travels further past the 4, 5 or 6 cushions, on a normally 'worn' table, NOT'new' cloth!
If you have Ceulemans book, it explains it, (not easily for a novice) but, to established players it's a common sense known fact.