The double clutch and/or that push button thing on the gear shifter. Not sure if that’s part of the same system.
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Double clutching is just a way to shift a big truck. Almost all automotive cars that have manual transmissions have what are called synchronizers, they are tapered brass or bronze pieces in between the different gears in the transmission. When you push the clutch in, and move the shift lever, those synchronizers speed up or slow down the gears inside the transmission, so they will sync together, and shift smoothly. A big truck does not have synchronizers, it just has steel sliding collars between the gears. It is up to the driver to speed up or slow down the gears, usually by use of the gas pedal, so those collars will slide together smoothly. When a driver double clutches, they push the clutch in a bit, not all the way to the floor, but just enough to disconnect power from the engine, then they shift to neutral, the let off on the clutch pedal. They then push the clutch again, and slide into the next gear, and let off the pedal. The part in the middle, when they let the clutch out while in neutral is designed to help get all the gears going about the correct speed. The second clutch in is designed to take the force off of everything so they will slide together. By design, double clutching is meant to put the wear into the clutch, a part that is designed to wear out, and is cheaper to replace than the internals of the transmission. Most professional truck drivers do not double clutch, they don’t use the clutch at all, except for stopping and starting. They just feel and float the gears from one gear to the next.
The button or lever on the shift stick is usually a range selector. Big truck transmissions are capable of anywhere from 10 to 18 different gear ratios. In the simplest terms, it’s two transmissions built together. The front half is shifted by the stick, similar to a regular car. The range section is usually one or two or maybe three positions, shifted by the lever or button on the shift stick. For example, when taking off from a stop, a truck will have the range selector in low range, and then will shift 1,2,3,4,5, then will switch the range selector to high range, and shift 1,2,3,4,5 again. Those second set of shifts are actually gears 6-10, but the shift handle moves the same way it did 1-5.
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