well Im not all that knowledgebale on synchromeshes tbh...
But as for the clutch - a clutch is simply two plates. One on the gearbox side, one on the engine side. Usually to change gear you push the clutch pedal in, to seperate the two plates. this then allows the synchro and the the rest of the box of tricks to change gear smoothly - i dont know how it does it, it could be magic for all i care as Im never gonna take a gearbox apart. but doing all that causes wear, as the synchro, the tailshaft, the engine and the two plates of the clutch will be moving at different speeds. take the foot off the clutch and *BAM* everything wears as it changes to the correct speed.
Leave the clutch in, and if you get the engine speed/road speed etc correct, you can change gear without altering the speeds.
Im sure i will be corrected shortly