Tussengas is the designation of a special system for reversing the gear of a non-synchronized gearbox. It is double disconnected. In general, this is often called "double cloths", according to the English term: "double clutching" (or double declutching).

The technology is only required when switching from higher to lower gear. When switching on, this system is not required. Then the circumferential speed of the gears on the secondary shaft and the main shaft is substantially the same and the gears can easily be pushed together (if the clutch is stepped).

When the gear is switched back, the circumferential speed of the gears is not equal. By providing intermediate gear, the speed of the gear is adjusted to the secondary shaft and the gears can be pushed together with a slight pressure against the gearshift.

No interchange is possible. Also, with very great force on the gearshift trade, it is not possible to slide the gears together. The car should then be stopped by braking.

When switching between the intermediate shaft, first the clutch pedal is pushed in, then the gear lever is moved to neutral mode, then the clutch is released again and gives an adjusted amount of "intermediate gas", after which the clutch is pushed in again and sets the clutch gear lever accelerates lower and causes clutch to rise again. The purpose is to match the gear wheels in speed before they interlock.

In synchronized gearboxes, the gear difference between the gears is captured / adjusted by synchrome rings that are subject to wear. As a result, a synchronized bin will always wear faster than an expertly serviced unsynchronized bin.

For passenger cars, double clutching has become unnecessary when synchronized gearboxes became commonplace in the 1930s. Nevertheless, passenger vehicles with a double clutching mechanism were manufactured until the late 1960s. A number of old-timers still running around like the Fiat 500 and the Morris (Model "Traveler") still has this principle, which requires a separate driving technique. Also for trucks and agricultural vehicles, gearboxes have still been used so far, this mode of operation is required.

In the case of lorries, unsynchronized gearboxes are used for a longer period. Due to the heavy gears, it was also necessary to apply a special technique of double clutching when switching on. Of course, no intermediate gas was required: one had to wait a few seconds before the gears of the secondary shaft had lost a lot of speed so that they could be switched without cracking. In descents, this waiting time had to be shortened, as the main axle increased by the increasing vehicle speed, but when climbing the waiting time had to be extended. That could result in the switch-off impossible: the waiting time was sometimes so long that the vehicle stopped now.

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