The entire position would be closed before earnings, so we wouldn't actually be there for the jump. Assume the underlying is unchanged or we're delta hedged against any price movements. The back month would see increasing implied vol as we near earnings, while the front month would see unchanged or decreasing vol. Wouldn't the strategy actually perform better if the jump is overpriced, since the back month vol would see a bigger increase as we approach earnings?This is called being long the jump. This strategy will lose money if the jump is over priced. The strategy is also short gamma and will lose money if the realized vol is greater than the ambient vol.