As to "what do they do". I wish I knew! That is I wish I knew where to purchase the tooling to do it as I would do it myself. They remove the u-joint (not hard I have done it numerous times - simplest method is to burn the center out with a torch and drive the caps inward). The original joint is held in by "crimping" the yoke. Once the caps are removed the crimping is reamed or honed from the yoke. At this point new u-joints can be assembled. They need to be held in though.
The beauty of the shafts Beyer does is that they then machine the yokes to accept snap-rings such as well built shafts were built before disposability. This machining is the trick. There is more to it than just a grove cut in the yoke (I really would like to see that machine). Getting it centered is absolutely critical to balance. Other than the this part all they have to do is make sure their tubes are straight and that the whole thing is balanced and indexed properly.
Bosch Master, ASE Master, L1
33 years MB technician