Below you can find a diagram and explanation of the Porsche System gearbox synchronizer as found in Paul Frére’s “Porsche 911 Story”. First used in the Cisitalia racer from 1946, it would later be found in all street Porsches, and through licensing, in many other cars around the world from different manufacturers.
If you take the time to “study” it, you’ll see that when the driver tries to engage a gear, until the gear speeds are matched, the synchro will oppose a force proportional to the force exerted by the hand on the lever. Thus, in normal conditions, it becomes almost impossible to force a gear before the “right” moment.
Competing and simpler (and cheaper) synchros from Borg-Warner couldn’t guarantee such safety. However, due to cost reasons I suppose, all manufacturers stopped using the Porsche synchros sometime ago, turning to the B-W design as, besides the lower cost, the B-W synchros also require less effort to engage.
Even Porsche, starting with the G50 box, also turned to B-W. The last box to use their synchros was the 915. But… there is something deeply satisfying in rowing a 915 box. There’s a feeling (a magic ?) when the gears smoothly mesh that you can’t find in any modern box, even Porsche’s… Try it.