I never get fills by them. I'm guessing you have to be a losing trader to be eligible for fills from them. From their website: What is the Timber Hill Automatic Execution Service for IB customers? Timber Hill provides an automatic execution service for certain eligible IB customer orders in certain Nasdaq stocks and NYSE and other exchange listed stocks. When IB's Best Execution order routing system searches the available market centers for your order, if Timber Hill is willing to provide an execution at the best available national price or better for that stock, IB may send your order to Timber Hill for an immediate automatic execution. How can I benefit from the service? If your order is eligible for automatic execution, you will get as good a price as any price available in the national market system and, most importantly, your order will get executed immediately, with no delay or chance of the market price moving away from your order before it is executed.