with double clutching , you depress the clutch to unload the gear you are in to kick it out of that gear and throw it into neutral, then you releas the clutch to allow the input shaft to match the engine rpms so you will be able to match the gear speed and be able to slip into the next ghear when the rpms match up , then the next cluth application , in theory allows a little extra slop to be ale to slip into the next gear.
Works great on paper. In reality, with a big truck, you're much better off just floating them all. The only thing double clutching does is double the wear and tear on clutch parts and your left leg. With a big truck about the only time you need the clutch aside from stopping and starting is on steep hills, up or down, a quick half stab of the clutch to unload the gear makes for faster shifts with less loading on the tranny.
Back when I learned to drive a big truck , it was trial by fire and this was pretty much the norm. The guy who owned the truck had the tire billy in his hand and told me how much a clutch cost for that thing and said "any time you step on that thing and you aren't stopping , I'll reach out of the bunk and whak yer left knee"
I'm pretty sure he ment it. Good thing I'd already had plenty of time in a straight truck with no synchros
Just the going from 5 to 20 gears was a bit mind boggling ( nothing like waking the boss up because you're doing 90 down a mountain and can't get the $%*&@ thing back in gear
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----Robert