When you 'shift' gears, with the exception of reverse, you are not actually moving any gears at all. All the gears are in constant mesh and rotating. What you are doing is moving a selector fork which moves a dog ring that locks the appropriate gears to the rotating gear shafts. The sychro ring matches the speed of the gear to the speed of the shaft such that the dog ring's splines can move to lock the gear to the shaft. The dog ring is held in position by a detent on the selector shaft. It is quite possible for the syncro ring to prevent the dog ring moving fully into position and the detent fully engaging as it is all part of the gear/dog ring assembly. Believe me the synchro/dog assembly CAN cause gears to 'pop' out, or rather prevent them from not engaging fully in the first place. I have rebuilt at least three 'boxes (Fiat RWD and VW) that have had this problem. However, I imagine that DC use the term 'synchro' to refer to the complete synchro/dog ring assembly which often comes as a pre-assembled assembly.