Originally Posted by T.Dot_E30
They work everytime you change gears,...they just don't have to work as much when the engine speed is close to where it should be.
The engine speed doesn't matter if you're not double clutching. The syncros are designed to speed up or slow down the input shaft of the transmission, nothing else. They're just cone clutches (usually brass) that help engage the dogs. That's why it's a terrible idea to shift without clutching on syncro gearboxes - if you're off by even a little bit, those little clutches have to slow down or speed up your engine and they are not designed to do that. And if they wear out, you won't be able to get into gear at all. I'd personally love a straight cut non-syncro box in my track car some day.
If you down shift and revmatch, all you do is make the clutch do no work, the syncros won't know the difference. For upshifting, you can make a difference by shifting quickly, so the input shaft only slows down enough to match the new gear speed and the syncro doesn't have to do any work.