How the diaphragm gets pushed. A dynamic driver glues a voice coil to a cone and parks it in a magnet gap; current through the coil makes a force at that one spot, and the cone has to carry that push out to its edges. A planar driver etches the conductor across a thin flat film hung between two magnet arrays, so the force lands evenly over the whole surface.

At low frequencies it barely does; the cone moves as one piston either way. The split shows up high, where the wavelength inside the diaphragm gets short. A point-driven cone has to propagate that motion outward, so the rim starts lagging the center and the surface stops moving as one piece. It breaks into modes. The flat film, driven everywhere at once, stays flat.

It is. When different parts of the diaphragm move out of step, they radiate sound that was not in the signal, which shows up as resonance peaks and extra distortion in the treble. Spreading the drive is how planars keep distortion low up high. It is also why a planar diaphragm can be so thin and light: it never has to be stiff enough to stay rigid under a point load.

Tradeoffs. Those magnet arrays are heavy and the big flat diaphragm is inefficient, so planars tend to be bigger, heavier on your head, and hungry for amp power. Dynamic drivers are light, cheap, efficient, and move a lot of air easily, which makes deep bass simple and lets them run off a phone. Most headphones in the world are dynamic for exactly those reasons.

That is the honest summary. Neither wins outright. A well-engineered dynamic driver can sound excellent, and a sloppy planar can sound worse than a good dynamic. The mechanism sets the tendencies; the tuning and the build decide the result.