Technology

I can't speak for big rigs, but I drive and ride multiple manuals. Synchro or not, there's no reason to double clutch an upshift in the cars under normal or high performance situations. It'd only make sense if I took too long to shift and had the engine rpm fall far below what matches the speed of the next gear. It's a drag race. They're burning synchros to drop 6krpm to 4k in the next gear in half a second. Even in normal driving, dropping 1000rpm or more is plenty of time to catch the next gear. 2 of my mini trucks have burnt synchros on one gear each (prior to my ownership), so I'm pretty well aware of how to time it for a smoother shift on the downfall. If double clutching was necessary on upshifts, I wouldn't be able to do gasless clutchless shifting. But I can

Downshifts, absolutely. There's plenty of reason to double clutch a downshift. The engine is, by definition, under spun for the next gear so yes, blipping it up will make it easier to drop a gear. Not needed for 1 gear at a time with good synchros, but certainly adds consistency when I do a 5>3 downshift to pass in the truck with a burnt 3rd. Almost required when I had braking problems and needed to downshift into 1st since the speed differential was far greater.

The source of the line form the movie is probably from the theatrical soundtrack from Bullitt. The engine sound was recorded separately from a GT40. The driver double clutched because it sounded mean.

Let's not forget the line comes from a scene in which granny shifting burnt the piston rings, dangered the manifold, made the floorboard fall off, and spilled a jar of o-rings.

I assume you used the wrong word towards the end. The flywheel is bolted to the crankshaft. If anything stops the flywheel, the engine is now turned off.