As you know, a U-joint has 2 yokes connected by a cross.

The yokes are 90 degrees apart. When you stab the slip

joint you are making 2 parts into a single shaft. On this

single shaft the yokes at each end need to line up exactly.

(look at the other shaft and see how they line up on it.

When you get the slip joint stabbed right, the 2 yokes on

that part of the shaft will be lined up the same way.)

This will minimize end thrust introduced by the angle at which the

2 parts operate, and thus also minimize endwise motion of the

slip joint.

If the shaft is properly made, luck is not involved.