Basically it refers to the yokes being in alignment with each other. In other words, the u-joint on the rear shaft slip yoke
must be installed onto the slip stub so that it is in the same position as the front joint or "in phase". Any other position will not transmit power smoothly and create an imbalance situation. That will probably mean loosening the ball connection at the tranny and checking the postion of the front yoke and then connecting the rear slip so that the joints are aligned.
A little hard to explain w/o a diagram or photo but I'm sure you'll understand once you take a good loook and think about it.
On these front enclosed two piece units it's wise to paint or mark a stripe on the slip stub and the slip yoke so that they go back togethaer as they came apart.
