A defrost microwave mode operates at a lower power setting than the regular microwave mode to thaw frozen food more gently and evenly. When you use the defrost mode, the microwave cycles on and off to prevent cooking the food and instead allow it to thaw.
The reason you can't immediately switch from defrost mode to regular heating mode is due to a safety feature built into most microwaves. Microwaves have an automatic cooldown period after using the defrost function to prevent potential damage to the microwave's magnetron (the component that generates microwave radiation) and other internal parts.
The cooldown period allows the magnetron to dissipate excess heat generated during the defrosting process and helps protect it from overheating. If you were to immediately switch back to regular heating mode without this cooldown, it could put additional stress on the magnetron and other components, potentially causing malfunctions or reducing the microwave's lifespan.
The duration of the cooldown period varies between microwave models, but it usually lasts for a few minutes. During this time, the microwave fan may continue to run to aid in cooling the internal components.
If you want to heat up food right after defrosting, you can do so after the cooldown period is over. Alternatively, some microwaves have a quick-start button that allows you to directly start heating after defrosting without manually changing modes, as it automatically transitions to the regular heating mode once the cooldown is complete. However, it's always a good idea to consult your microwave's user manual for specific instructions and safety guidelines related to its operation.