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I haven't needed to do much of this, but I'll do my best. On tractors and an old MG convertible we had this was necessary. If you're rolling along in first gear, you press the clutch in, and move the stick to neutral. You let the clutch out. You put the clutch back in and shift from neutral to second and let the clutch out. This as compared to a "normal" tranny where you put the clutch in, move from 1 to 2, and let it out.

Modern cars have good enough synchros that this is not necessary. I'm not sure if modern big rigs still need to do this, but I rented a big u-haul a few years ago and it was necessary to keep the gears from grinding.

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one thing must be added!
when you push the clutch in you rev the engine a bit so the speed of the engine synchronizes with trannys next gear(whether it is up or down)
This technique is no longer needed with manual trannys with the advent
of sychronizers on modern gearboxes.

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The original reason for double clutching was dealing with 2 cycle engine because the RPM's drop drastically fast after the gas pedal being released and the clutch applied.

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