I'd be hard pressed to explain it as well. That's the problem. I did a search and found a similar sentence that in fact uses "when" in that form. It was on Google. I didn't save it. I'd have to find it again. Using "when" somehow occurs to me as a way to invert the sentence, but I understand your point about it not sounding completely correct. Still, it doesn't sound completely incorrect to me either. Maybe I'll do another search or check the Collins Concordancer later. Of course, it wouldn't be conclusive, but at least it could be seen as some sort of indicator.