The initial starting function begins with the power aplied to the winding of the solenoid. Part of the ground for this circuit goes through the brushes and armature. As the solenoid coil becomes a magnet the plunger is pulled back. This action shoves the gear into the flywheel. When the plunger gets to the end of its stroke it switches the high current switch that carries the voltage from the battery cable to the heavy lead into the starter. The point to this is that if the starter is spinning the plunger has plunged and thats all the solenoid does.
There are three possibilities: first is the fulcrum bolt is loose in the starter, second the one way clutch is freewheeling, and third there is a section of gear missing.
Bosch Master, ASE Master, L1
33 years MB technician