What is "vacamatic"

The 1941 M4 Vacamatic transmission by Chrysler was, historically, the first semi-automatic transmission which was marketed by a major manufacturer. It was an attempt to compete against rivals' automatic transmissions, though it still had a clutch, primarily to change range. In normal driving, the clutch is not used. The transmission itself was a fully synchronised manual type, with four forward gears, one reverse; where the shifting was done 'automatically' by either vacuum cylinders (early, M4), or hydraulic cylinders (late, M6, Presto-Matic). The main difference was the addition of a fluid coupling between engine and clutch, and the shifting mechanism.