The microwave defrost setting on your microwave works by cycling the magnetron (the microwave component that generates the extremely high frequencies that heat food) on and off.
As a result, microwave power is reduced to between 20 to 30 percent. In contrast, microwave settings that use higher power levels allow the magnetron to generate frequencies continuously, which means the appliance is running at 100 percent power. On some microwave models, you can indicate the weight or type of food you’re heating to calculate the appropriate defrost time.