When you install a u-joint, it has to find the correct location width wise based on the distance between the grooves for the snap or retaining rings. The ends of the trunnions ride against thrust washers in the caps to operate correctly. After the u-joint is installed in the yokes, the easiest method to ensure that the spacing is adhered to is to properly smack each yoke on both sides just below the hole with a decent size hammer. That uses inertia and the weight of the opposing yoke and u-joint body weight to set the cap back snugly against the snap or retaining rings which also then allows the joint to operate with the proper amount of preload on the thrust washers.Please elaborate on what you are trying to say.
Driveshaft joints require less forceful smacking to behave than alloy axle shafts. Kinda like the difference between your girlfriend and your wife and knowing which one to smack and how hard.
If one will move the opposing yokes before and then after while paying attention to the amount of force to do so, a significant freeing up of the joint can be felt which tells you that it is now correctly installed. Inertia, learn it, use it to your advantage.