With the short calendar earnings play, you are paying for a very very expensive gamma, so you need to short a contract in the back months that is very very rich in vols so that a flat open doesnt hurt you. Every now and then when the tenor skew is relatively flatter, you can establish a vega neutral position, which means you break even with a flat open. And no it is not an arb, you still have to call the magnitude of the vols collapse right. In this case, any move will yield gains due to your long gammas. That's what iv was talking about.
