Sorry, but the left pedal was not a clutch. If you used it as such you got into problems, which was a common preselector issue for those used to conventional transmissions.
The type of buses used in London until the 1970s (the RT type) had these fitted, and you could watch the differences the driver had to do through the window behind. There was a hiss every time the pedal was pressed as it was air-powered. At the bus stop, flick it into second (first being very low), stamp on the pedal, flick it into third - and all this before the bus started, for which you just pressed the accelerator.
Lets get back to aircraft. I have a question about piston engines. The Centaurus engine (Airspeed Ambassador and others) was a sleeve-valve engine. How did these work ? Was this behind the huge cloud of smoke on startup after standing overnight ?