We report on the electrical detection of spin resonance in a two-dimensional electron system modulated by a periodic magnetic field with zero average. Spin degeneracy is lifted by a large magnetic field applied in the plane while the system is irradiated with microwaves. Without magnetic modulation, the resistance does not detect spin resonance. However an absorption peak develops as the magnetic modulation is switched on. The frequency and temperature dependences of the peak yield the Zeeman energy of electrons in the GaAs/AlGaAs quantum well. We interpret the absorption peak as the result of competition between two spin-flip transitions: one activated by snake orbits oscillating at the boundary between positive and negative magnetic field domains, the other by microwaves. When both transitions are simultaneously resonant, the system forms a dark state which blocks spin flips and freezes snake orbit channelling. The coherent suppression of snake orbit channelling explains the experimental features of the observed resonance.