Similarly, if two exchanges agree to show their order books to each other, a couple of seconds before they are posted to the public, they can exploit all opportunities for arbitrage between them. Any client who tries to do arbitrage will then find that the prices have always shifted against him between the time that he places an order and the time that it gets executed.
Easy for an experienced trader to detect this, as they watch the price on multiple exchanges at the same time.
How exactly? To the people watching the logs, those insider trades would be indistinguishable from normal trades.
Without the insider trades, an outsider would occasionally see a difference in price lasting long enough for him to exploit. With insider trades, there would be fewer such opportunities; and when one appears, before the client can execute his trade there would appear another trade exploiting the difference and eliminating it.
Specifically, for example: the outsider sees that on exchange A the highest bid is 450$, on exchange B the lowest ask is 440$. He submits a buy order on B for 440$, but before it gets executed, someone else buys that offer; his own order just lands on the book, unfilled, while the lowest ask on B has jumped to 455$. How can he tell that the buy that succeeded was insider arbitrage, rather than a normal trade at B, or A-B arbitrage by a luckier competitor?