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Well, whippersnapper, just sit down right there and I'll tell you a story. Once upon a time, long ago, there were no automatic transmission and what primative gearboxes there were had no synchros. (Gasp!) Yes, its true. To get from a higher gear to a lower one you had to either shh..iff...tt real s..l..o..w to avoid raking the gears or you could do a nifty double clutch. You depress the clutch pedal to disengage the gears, let it out and quickly rev the engine to cause the lower gear shaft speeds to match the speed of the shaft you are moving from. With proper timing and experience, you can again depress the clutch pedal (double clutch, see?) and the gears will mesh without grinding or raking. The advantage is the reduced time to shift, especially when racing. Practical applications include being able to drive home when the clutch fails. It works for upshifting as well as down.
Even with syncros it allows you too down shift faster with less wear to the parts. Still necesary in most real haulin trucks on down shifts as most tandem axle trucks don"t have syncros. Even helps with gear speed matching on upshifts if that is your style of drivin.