It allows convenient driver control of gear selection. For most of automotive history, automatic transmissions already allowed some control of gear selection using the console or shifter, usually to limit the transmission shifting beyond a certain gear (allowing engine braking on downhills) and/or locking out the use of overdrive gears when towing. It enhanced such features by providing either steering wheel mounted paddle shifters or a modified shift lever, allowing the driver to enter a "manual mode" and select any available gear, usually in a sequential "up shift/downshift" manner.
Some transmissions allow the driver to have full control of gear selection, though most will intervene to prevent engine stalling and redlining by shifting automatically at the low end and/or high end of the engine's normal operating range. Hydraulically-coupled and most clutch transmissions also provide the option of operating in the same manner as a conventional automatic transmission, by allowing the transmission's computer to select gear changes. A few also allow an alternate automatic mode, often called "sport" mode, where gear selection is still fully automatic but the transmission will favor higher engine speeds (at which the engine will produce the highest horsepower and/or torque) by up shifting later when accelerating and downshifting earlier when slowing.
A clutch-less manual facilitates gear changes by dispensing with the need to press a clutch pedal at the same time as changing gears. It uses electronic sensors, pneumatics, processors and actuators to execute gear shifts on input from the driver or by a computer. This removes the need for a clutch pedal which the driver otherwise needs to depress before making a gear change, since the clutch itself is actuated by electronic equipment which can synchronize the timing and torque required to make quick, smooth gear shifts. The system was designed by automobile manufacturers to provide a better driving experience through fast overtaking maneuvers on highways. Some motorcycles also use a system with a conventional gear change but without the need for manual clutch operation.
In the 1930s, automakers began to market cars with some sort of device that would reduce the amount of clutching and de-clutching and shifting required in stop and go driving. Most typically, a fluid coupling or a centrifugal clutch replaced the standard manual clutch to allow for stop and go driving without using the clutch pedal every time the car was brought to a stop. More sophisticated systems allowed for shifting while driving without using the clutch, and some systems did away with the clutch pedal altogether. Semi-automatic transmissions were phased out as technology advanced and automatic controls were developed to take care of changing gear ratios. Smaller, lower powered cars used semi-automatic transmissions with a dry clutch because the mechanical connection offered a more efficient powertrain compared to a fluid coupling.
Another early semi-automatic transmission was the Sinclair S.S.S. (synchro-self-shifting)
Powerflow gearbox. which was applied to Huwood-Hudswell diesel mines locomotives. It was also applied to some road vehicles.
Improved semi-automatics appeared in the 1950s and 1960s. The Automotive Products Manumatic and Newtondrive systems were also known as "two-pedal transmissions". They relieve the driver of the need for skill in operating clutch and engine speed in conjunction with the gear change. The Manumatic has a clutch servo powered by the vacuum at the induction manifold operating the automatic clutch - a conventional clutch incorporating centrifugal operation. A switch in the gear lever operates a solenoid valve so that when the gear lever is moved, the clutch is disengaged. A control unit made throttle adjustments to keep the engine speed matched to the driven clutch plate and also varied the speed of clutch operation appropriate to road speed. The Newtondrive system differed in making a provision for choke control and a cable linkage from clutch operating mechanism to the throttle. These systems could be fitted to smaller cars such as the Ford Anglia.