The deadman or “racing throttle control” was developed for hydroplane and runabout racing boats. You see, when raced, most if not all the courses only had left hand turns. Having the throttle on the port side also gave the driver something to hold on to when making those fast turns. He can hold on to the throttle and also the steering wheel. The “deadman” part of it was that the throttle lever was spring loaded and if the driver let go of the handle (like being tossed out of the boat) the throttle would spring back to idle. Most racing motors back then, and now, call for a closed butterfly which means that when brought back to “idle” the motor would shut down due to the carb butterfly fully closing and shutting off the fuel/air.
.
.