Here is a view of the inner workings of a Bosch starter solenoid. An ingenious piece of electronic gadgetry, it functions totally contrary to what my tired old mind has believed for years.

Here you will see what really goes on inside a solenoid. In this first photo, the left part or slide as I would call it, is the part that pulls a lever that engages the starter bendix or 'drive gear'. The spring to the right of that is what pushes the slide back to disengage the starter gear once the ignition key is turned to 'run' position. The coil assembly at the top is actually TWO electromagnetic coils in one: Both receive positive current from the starter switch via terminal #50 when turned to 'start'. The 'pull' coil receives negative or ground through two of the starter brushes. (This is where the discussion veered from what my mind 'had always know as fact'). The second coil is a 'hold' coil, this is grounded through the solenoid case. The pull coil's negative side reverts to positive at the very moment the starter motor begins to turn and the contacts bridge, this increases the hold position's magnetism so that the gear does not disengage prematurely. When power is dropped to the 50 terminal both sections of the coil lose magnetic power and the slide is relaxed, disengaging the starter. 

Here we see the contact area. The large copper bar is the aforementioned bridge. You can also see clearly here that the 50 terminal has dual coil wires attached, and that one coil grounds to the case and one to the bridged terminal.  

Pretty wacky, but this works.

Many thanks to Al Brase along with Gnarlie to prod me along into discovery and re-education mode. :-)

You're never too old to learn.