"This auditory illusion allows you to easily switch between the two words: Brain Storm and Green Needle, depending on whichever word you're thinking of. It's even possible to hear an amalgamation of both words, such as "brain needle" or "green storm."
Brain and green sound similar so you'll hear the one you're waiting for. Storm and needle, well you can hear green storm or brain needle if waiting for it. I suspect expectation allows you to ignore sounds you are looking for and note those you are.