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There has been much debate about the original source of this technique. If not the outright inventor, one of the earliest applicants was Rauno Aaltonen, a driver whose approach to rallying and cars was so analytical that he earned nick-name "the Professor".
Before advent of four-wheel drive and means to affect both the front and rear wheels in different manner via differentials of modern rally cars, left foot braking was a solution to handling problems of front-wheel driven cars. When driving around a bend, initial braking tended to force the car to oversteer and subsequent acceleration to understeer.
To combat this, in left foot braking driver controlled the rear wheels with brakes and front wheels with throttle. This of course required brake balance to be shifted towards rear, somewhat unorthodox approach to a car set-up. Net effect was that under braking driver retained control with steering wheel as only rear-wheels locked and at the same time rear end of the car "came around".
It is noteworthy that while technique was originally developed for front-wheel driven cars, some drivers tried and did use it with rear-wheel driven cars too. 1
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After revolution of four wheel drive and, more importantly turbo engines, left foot braking found new use.
Four wheel driven cars employ differentials to split power of engine to front and back wheels (and also to left and right, like two wheel driven cars). Unlike in the earlier cars, with all-wheel driven cars the brakes were no longer the only way to influence the car's front independently from the rear.
At the same time, turbo-charged engines became the norm and with them, turbo lag. As is known, engine revs up faster than the spinning blades of a turbo accelerate when throttle is pressed. This slight delay is known as turbo lag. In modern rally cars, various anti-lag systems are used to combat this phenomena but with early cars, it all was up to a driver.
Therefore left-foot braking, which had been used for quite different purpose, found a new use. Drivers kept engine revving while braking and, subsequently, turbo spinning and boost up. Instead of lag, power was there immediately after the corner.
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1. Davenport, John: Rally (Hamlyn, England 1977), pp 80-82
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