While double clutching[1] is not necessary in a vehicle that has a synchronized manual transmission, the technique can be advantageous for smoothly downshifting in order to accelerate and, when done correctly, it prevents wear on the synchronizers which normally equalize transmission input and output speeds to allow downshifting.
Before the introduction of transmission synchronizers in 1927,[2] double clutching was a technique required to prevent damage to an automobile's gearing during shifts.
Due to the difficulty and most often unnecessary redundancy involved in the technique, coupled with the advent of synchronized gearing systems, it has largely fallen into disuse in light vehicles.
According to the National Highway Traffic Safety Administration (NHTSA), municipalities across the United States have banned the use of compression-release engine brakes, which includes jake shifting, because of noise emission.
If the speeds are not matched, the dog teeth on the collar will "clash" or grate as they attempt to fit into the holes on the desired gear.
Double clutching, although (slightly) time-consuming, ease gear selection when an extended delay or variance exists between engine and transmission speeds, and reduces wear on the synchronizers (or baulk rings), which are brass cone clutches themselves, and wear very slightly each time they are used to equalize the transmission revs with the output revs.