[BK]

I haven't looked at the photos linked above, but most synchros I've seen in photos are pretty unsophisticated. They look like brass rings. One side is fixed to the collar and the other is open and grabs the end of the spinning gear to synch up the speed of the collar to the gear, so the teeth of the collar will plug into the gear without grinding. Basically, it is grabbing the gear without teeth to allow it to slip a bit and speed up, before it inserts the teeth into the gear.

Those with more knowledge can correct me if I've described it wrong, but it seems like a brilliant but uncomplicated solution. I'd imagine synchros get hot quickly and will get worn if you do a lot of downshifting without rev-matching.