That all makes sense, but I'm still struggling with how the...

That all makes sense, but I'm still struggling with how the share price will fall as a result of XXX buying and ZZZ selling when both are controlled by me (aside from the fact that the rules are clear that if the ultimate beneficial owner of XXX and ZZZ is the same person, then the trade is not allowed, but lets assume that someone is trying in on anyway). The rest of your narrative makes sense insofar as when stops are hit, some people are forced to sell and that may enable anyone (not just a shorter) to buy at a price cheaper than they otherwise would...

It's just that I genuinely can't see why a crossing would cause the price fall that kicks this all off. I'm buying 1m shares from myself... that makes the price fall? Alternatively I'm selling 1m to myself... makes price fall? I have to be on both sides of the order book for this to happen - artificial supply of stock but also artificial demand. I have seen large crossings induce additional volume - that is, abnormal volume appears and that creates interest in the stock which wouldn't have been there in the absence of the crossing, but the subsequent direction is rather unpredictable.