It's basically spending all the muni one could have all at once. I didn't see much destructive effect, though- a lot of suppression, that's all.
In the trailer, there was this triple stuka attack with strafe AND bombing.
I was also suprised that the ability didn't cost any manpower, but 250 munitons is a LOT for OKW. And its 3 planes coming in to strafe, two from the north, one from the south. It has a huge area of strafe for a pretty long duration.