With the inertia starter motors they power was added and the ring gear would spin at full speed. The gear would then spin inwards due to inertia along the grooved lines and hit the flywheels gears at full speed. This was such a harsh event that it would grind, break or mangle the ring gear and flywheel teeth.
Pre-engage starter motor:
when you engage the starter motor the pinion gear gets pushed out onto the flywheel, this step is the stop that violent event of the pinion gear and flywheel meshing. This has the negative effect of the starter needing a lot of power to overcome its own inertia and then the inertia of the motor.
