no what happened yesterday is irrelevant to what this conversation is about.
also about that, most of what sold off came back from that sell off within 10 minutes or so, after ap came out and said they were hacked.
what you are referring to are ECN's,
electronic communication network ,
they are for orders off the exchange, they create a lower cost and access to exchange's books on a specific instrument.
what this conversation is about are systems created by so called professionals to supposedly create an effort less strategy of trading and other categories mentioned , but they are statistical based and fail once the statics change, which is quit often.
and no, no example,
I'm a series 3 capital markets trader. i'm doing it at this second.
i tried to provide useful info about these systems,
9/10 these systems will blow out your account.