So how exactly does the "defrost" function work differently than "cook" on a microwave?

I’d guess that they just have different power settings and time settings which the makers have decided are best suited to those tasks.
In the end, you’re just heating something up with a microwave, there is nothing special about what those buttons do. I’m pretty sure you can achieve the same result by manually setting the time.

the defrost button causes the waves to be shut off periodically. It is exactly the same as using the different power levels. With a noisy machine like mine, you can hear it come on and shut off.

Some new units have a pulse-width modulator—a hefty electronic circuit that clips the power to the transformer, which lessens the power of the microwaves.

If it is not a variable power (Inverter) model it will use a series of very short, low intensity bursts, not enough to cook, just to warm. With an inverter model it simply drops the power level.

ive always wondered this….and behold the truth which is less satisfying then the actual action of pressing the button….sigh…

yeah as mentioned before, it’s a very low power setting followed by an even lower power setting, it doesn’t cook whatever you’re defrosting, it just raises it’s core tempature to 33 degrees F.(if you have one that works properly) Microwave’s are nifty little bits of technology, how they produce heat is very impressive mechanically.

This sounds complicated. I think I’ll just stick with the sink/ counter idea. Hah…