It's being started with an electric inertia starter but the hand crank is not directly related; it's below the magneto switch and is part of the ignition system.
Before magneto impulse couplings that provided a decently strong retarded spark for starting, mags used a separate primary ignition circuit with its own retarded-timing ignition points, for starting only (the normal points operate with way too much advance for starting), which didn't run off the magneto's internal generator and had to be powered externally.
The most common system was called a vibrator, or "Shower of Sparks" (a trade name) ignition system, which used an oscillating current device that would send shots of current through the primary coil to generate a succession of sparks starting at about Top Dead Center of a cylinder when energized from the battery. When the engine was running, the starting circuit was simply inactive.
The system in the video is similar but runs off an independent Permanent Magnet Generator turned by the crank to make sparks that are suitably retarded.
So what you are seeing is the engine is cranking, the normal magneto circuit isn't really making any significant spark because it's spinning too slowly, but at the right time the little cranks is spun, which will supply sparks, through the separate primary starting circuit, to light off the engine, and as soon as it's running, the normal mag circuit takes over. That's why it cranks, but doesn't fire until the crank is spun.