Basically it is using EMF Waves, at high frequency, and causing the Atoms to spin - the Protons, Neutrons, and Electrons - their Magnetic Dipoles will spin to the frequency of the EMF wave when in resonance and produce heat.
Simpler term is that Eddy Currents are induced to counter the effect of magnetism induced by changing EMF field as per Lenz Law, and Faraday's Law of Induction. But these laws were invented when Quantum Mechanics was not well understood. Same thing about the Hall Effect and Amperes Law.