Brother Reg elegantly steps us through the circuit diagram for this one.

When the Defrost cycle starts (Timer Motor pauses),

the Neutral goes through the Defrost Heater, Defrost Thermostat, and Thermal Fuse to Line.

Whenever the Defrost Thermostat "opens",

the Neutral still goes through the Defrost Heater, but now travels through the Timer Motor

and "seeks" the Line through the Compressor.(and Overload).

Motor resumes, but since the Motor is of a "high" resistance, the Heater hardly heats anymore,

until the Timer completes it's "Defrost timing", and then it switches the Neutral back to itself and to the Compressor.

If any of those parts aren't working, the Motor may not run at the specific time.

Source: Help Me Read This Wiring Diagram