Planar magnetic drivers use a thin, flat diaphragm with embedded conductive traces, suspended between arrays of magnets. When an audio signal passes through the traces, the entire diaphragm moves uniformly in the magnetic field. This is fundamentally different from dynamic drivers, where only the voice coil at the center of the cone moves.
The advantages: faster transient response (the diaphragm starts and stops more precisely), lower distortion at high volumes, and better bass extension because the driver moves as one unified surface. The trade-offs: planar magnetic headphones are typically heavier, bulkier, and less efficient (they need more amplifier power).
Historically limited to expensive audiophile headphones ($500+), planar magnetic drivers are now available at accessible prices: HiFiMAN Sundara ($279), Audeze Maxwell ($299 gaming headset), and even some budget IEMs now use planar technology. The improved detail retrieval and bass control are immediately audible compared to dynamic drivers at similar prices.