The institutional orders get baited to BATS BYX with "taker rebates" (on BZX the fee structure is the normal maker-rebate taker-fee, but on BYX it is reversed). It hits BYX first because the broker's router is programmed to send it there first for that rebate, but its only for a minimal amount of shares. And that's a signal which the HFT firms use to pull or front-run equivalent offers on the various other exchanges.

The other big scam seems to be HFT firms buying a broker's order flow - for hundreds of millions of dollars. For example, Schwab sold its order flow to UBS for 8-year contract in 2005, for $285 million. It was massively underpriced, Schwab is said to have left a billion dollars on the table. And UBS, in turn, sold Schwab's order flow to Citadel for an undisclosed sum. Why is the right to execute a broker's order flow so valuable? Nobody seems to have a good answer, but the obvious reason is that its easy scalping.