GM starters use a round solenoid mounted to the starter housing. The high starter current is switched on and off by a copper contact disc that is engaged by a movable core, called the plunger, inside the pull-in and hold-in windings. The plunger also forces the drive gear into mesh with the ring gear on the flex plate or flywheel.
Voltage is applied to the smaller solenoid terminal when the starter relay is turned on. Current flows through two coils of wire that together produce a very strong magnetic field. Those coils are called the "pull-in" winding and the "hold-in" winding. That magnetic field draws in a movable iron core called the "plunger". Two actions take place when the core moves. The mechanical action uses a lever to push the drive gear into mesh with the ring gear on the flex plate or flywheel. The electrical action uses a copper disc to connect the large "battery" terminal to the two pairs of field windings. The entire circuit is shown in Figure 1.
Figure 1. Starter circuit turned off.
Figure 2 shows the ignition switch and the neutral safety switch closed, (turned on). This is the low current circuit shown in green. The starter relay is energized so current flows from the battery, through the starter relay contacts, then through the two coils in the solenoid. That is the medium current circuit shown in blue. Current goes through the "hold-in" winding, then to ground. Additional current goes through the "pull-in" winding, then through the two field coils and armature, and to ground. Because the field coils and armature are so extremely low in resistance, in effect, the pull-in winding is grounded. It takes the combination of the magnetic fields from both coils to move the drive gear into mesh with the ring gear. When that happens, the contact disc completes the circuit between the battery terminal and the field coils.
Fig. 2 Starter circuit turned on
The last diagram, Figure 3, shows the starter motor operating. Now that the solenoid plunger has forced the contact washer to complete the circuit, the high current flows from the battery, through the contacts, then through the field coils and armature to ground. That high current circuit is shown in red. There's one more important detail to note. Both solenoid coils still have battery voltage applied to their common point, but now there is also battery voltage applied to the other end of the pull-in coil. With the same voltage applied to both ends, the net difference is 0 volts so current stops flowing through it. It takes the magnetic fields from both coils to move the plunger and engage the drive gear, but the hold-in coil's magnetic field is strong enough to hold it engaged by itself. Switching the pull-in coil off makes that current, as much as ten amps, available to the high current circuit. With a cold engine or weak battery, that ten amps might be just what is needed to get the engine started.