[greenshackle2]

The NBBO ensures you get the best price available on public exchanges. Public exchanges are not the only places where stocks are traded.

If the best price of a stock on exchanges is $25.00, but an HFT can buy it at $24.99 on a dark pool, they could buy it cheaper and sell it to you, and pocket the difference. When they pay your broker for the privilege of doing so, it's called payment for order flow. As a retail trader you have no visibility or access to dark pools.

Aside from dark pools, NBBO updates have latency which may be exploited:

http://strategicreasoning.org/wp-content/uploads/2013/02/ec3...

Not matter how they do it, the fact that they are willing to pay your broker to execute your order is proof that they have some way to make money from your orders.