With a starting crank on a car you are actually turning the engine directly. One turn of the hand crank = one turn of the engine.
With an Inertial Starter you first spin a small flywheel up to a very high speed, that's why it takes ages before they pull the crank handle out and try to start the engine. One turn of the crank handle = many many turns of the flywheel. The pilot engages the Inertial Starter in the same way as an electric starter, but there is no drain on the electrical system, it's just stored kinetic energy. Electrics weren't as refined in the 1940s and an Inertial Starter was a good solution, also lightweight considering it is turning over a 35 litre V12 piston engine!