system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is my microwave doing differently when I set it to defrost mode instead of regular heating?

Just occurred to me today I've been using the defrost button for 20-something years to quickly thaw meats, but have absolutely no idea how it makes a difference.
gpt
Depends on the microwave, but most of them basically turn the microwave on and off, more off time than on time, to reduce the amount of heating taking place over time.  The intent is to allow the item your cooking to warm up more slowly, otherwise you'd end up with a burnt exterior with a frozen center.  The outside of whatever you're thawing is where the microwave has its greatest effect while the center doesn't get much direct heat and instead relies on heat to be conducted through the food.  So it zaps the outside just enough for it to be warm, takes a break which allows that heat to pass on to the colder parts of the food and then zaps it again and repeats this pattern.

Some higher end microwaves instead decrease the intensity of the radiation used; Panasonic "Inverter" microwaves are an example.