I have a rough intuitive grasp of how they work, but it does not matter here. When you put in the clutch and change the gear selector from Third to Neutral, the layshaft is going a particular speed that matched Third gear at the present road speed. Ideally, layshaft has coasted down to exactly Second gear speed Let out clutch System is all locked up in Second. This is double-clutching. The rev-matching is the tricky part.
Experience is the only teacher.
Yes, it makes the synchros work less hard than they were designed to, so they will last longer. The synchros will make up for any errors. This works exactly the same in a synchromesh box as a crash box. Since the layshaft is always connected to the engine, synchros will NOT help you. In fact, I think errors in this method can wreak your synchros.