Hello Greg, When answering someone, I usually like to leave a few sentences of their remarks but as you can see above, your post disappeared when I opened your diagram and that was all I was left with. Bit of a pain but never mind.
This circuit for a motor speed control or light dimmer on this website is similar to yours and simpler.There is description on how it works. Perhaps you can study the diagram with description and then get a better idea how your circuit works, since some areas are similar.
I have built that particular controller for a DC motor in an old mig welder and it works very well. Much better than the original motor controller which justed varied the voltage to the motor. Just thought I would mention that if all you were looking for was a simple motor controller.
Regards, John Crighton Hornsby