How does the Polymer-Resettable-Fuse effect work?

At normal operating temperatures, polyethylene tightly wraps the conductor in a crystalline structure, forming a low impedance conductive path as shown on the left,because of the low impedance, the current flowing through the PPTC component on the circuit generates little thermal energy which will not change the crystal structure of polyethylene.

When the abnormal current occurs, the thermal energy generated on the conductor will change the polyethylene from crystalline to Amorphous, as shown on the right. In this case, the wrapped conductor will be separated by the expansion of polyethylene, leading to a rapid increase in impedance and limiting the abnormal current flowing through the PPTC component.
After the abnormal current disappears, the conductor path will combine again into low impedance.