I believe that the rule of thumb is to rack to secondary after fermentation is around 2/3 complete (if you are going to use a secondary/bright tank).
The reason, as I understand it, you still want SOME fermentation activity in the Bright. Now, as for the scientific reason, I am not sure. I think if you rack to Secondary/Bright AFTER fermentation is COMPLETE, the movement could cause a second fermentation which is bad (I believe). If you rack to Secondary/Bright while fermentaion is mostly complete but still active, the fermenation process will complete in the Secondary/Bright which is what you want.
Again, I am new to brewing (One BB IIPA Kit thus far), but have read alot and had this same question.
Best of luck!