The clearing firms essentially trust us to handle our own pre-trade risk controls.Quote from onelot:
how does your firm handle pre-trade risk? do you get a fixed allocation, and then are limited via a software routing library? or via a hardware check?
There's many ways to handle the checks, but in general it's done via your own code and works as you would expect (MinPNL, MaxOrderSize, etc, get checked against your orders and the market).
This is where exchanges/regulators could make some welcome and publicizable changes to HFT - by enforcing these checks at the exchange level to prevent fat finger trades and Knight-like fiascos.