I've designed and built a few ball shifting transmission in the past. I really dig the new Vex Pro models that are out. They've got many of the 'wanted' features I've recognized over the years, but never implemented. Kudos to them for a solid design.
Linked are two pictures.
1)
Project cover page listing team members and showing completed drive base.
2)
Shifting detail on the transmissions. The large exploded view is a bit crap (my apologies), but the other simple illustrations outline function showing the shifting shaft, plunger shaft, shift gears, and ball bearings.
I'm happy to share any specific details if anyone is interested in developing their own ball shifter.