Rocker movement is only part of it they have to have lash (gap) at the right time for compression to occur, compression is required for combustion, especially in a glow engine. The combination of compression residual heat and the reaction od the methanol with the platinum coating on the glow plug coil is what makes it light. Four stroke glow engines are even harder to make run due to the extra cooling during the extra rotational cycle that two strokes don't have.