Microwaves vary by make and model, but some have sensors to determine when the popcorn is thoroughly popped. Sensors include a microphone to detect when popping slows, and a humidity detector which determines the humidity at which popcorn has neared completetion.
Most brands of popcorn advise against using the popcorn button because of this variation in models. Models that have a Popcorn button without a sensor merely turn on the microwave for a fixed time, which may result in burned or undercooked popcorn since different brands of popcorn have varying microwave time requirements.
