The Bendix gear is driven on the shaft of an electric motor. When working properly, the gear locks on the shaft in one direction as long as it is rotating in that direction almost like a socket wrench ratchet. And like a broken ratchet handle, when a Bendix gear starts going bad it will simply 'let go' when it's supposed to be locked.
The motor armature is hefty and spinning fast. Of course, the motor will keep rotating even after power is removed, simply from kinetic energy.
When the engine starts, the Bendix is still engaged but the flex plate starts going faster than the starter motor, still rotating in the same direction so the ratchet direction is still satisfied. Since the flex plate is going faster than the Bendix gear, the gear stays engaged but it free-wheels until starter motor power is removed.
When you let go of the key, power is removed from the starter motor. Rotation slows, the Bendix gear disengages and returns to the 'home' position, ready for the next cycle. If your starter motor doesn't perform as described, it needs maintenance.
Your starter motor brushes come in two sets of two (four all together). Two brush pigtails simply bolt onto the case which is grounded to 'chassis'. The other two are soldered under a folded piece of metal. You will need a large soldering iron to remove the old and replace with the new. Original soldering irons were simply held in a fire until hot. They were made of iron to hold heat. You can heat a hunk of steel with a propane torch to accomplish the same thing. The iron should melt lead but red hot is too hot. Touch the connection with your home made iron and pull the old pigtails out with a pair of pliers because the connection is crimped. Reverse the procedure for the new brushes but add a little more solder after you crimp it.
Make sure your new brushes slide easily in their brass holders. They need to be smooth with no obstruction. I simply bend a short length of coat hanger on the end and pull the pigtail up and down as I watch the spring tension pull the brush back down.
When done, this job is very rewarding and you will save a bunch of money. Take lots of pictures as you go. - Dave