I haven't been inside an SR box, but most manual trannys will be similar. The blocking ring (I learned it as "baulking ring" from MG manuals) slides over the synchronizer and locks it to the gear. It snaps into place and is held by a mechanical detent. If it doesn't travel far enough (bent fork or worn fork), then it doesn't lock into place. It will transmit some torque but jump off when the load gets too high. That is why the wheels turned in the air, but not on the grouind.
Incidently, driving with your hand on the shift lever can sometimes keep pressure on the fork and "blocking ring" to the point where the fork will wear out. Also, if the box did not have GL-4 oil in it, the fork could have worn.
Incidently, driving with your hand on the shift lever can sometimes keep pressure on the fork and "blocking ring" to the point where the fork will wear out. Also, if the box did not have GL-4 oil in it, the fork could have worn.