How does the advancing wing in a flat spin create nose thrust?
A very good question, considering so many people do not draw the thrust vector for gliders.

The wing generates thrust by converting energy from falling to forward motion.  From scratch building gliders (and reading) one comes to realize that a falling parachute (straight down) only uses drag to reach a constant rate of descent.

A glider, with its center or gravity off set from its center of pressure (including tail!!!) will start to move sideways (glidogenesis) and generate lift with its wing.  This motion, known as "thrust" in this example, uses altitude as fuel and falling as its engine.

Notice both wings may be "thrusting" as it corkscrews down, but the outside wing, with its lower AOA, generates more lift/thrust and much less drag, sustaining the spin.

Once yawing is stopped, the wings equalize, and can be unstalled by lowering angle of attack.