I am having a bit of difficulty understanding information from IB documentation and from IB employees, as to the logic IB uses for its "last price" method for triggering stops. Maybe IB customers with more experience in this area of IB would be able to give me a better understanding. My equity trading, in the past, has pretty much steered clear of IB equity stop orders, for example, by using conditional orders triggered by futures contracts, so that my knowledge of IB's equity stop orders is limited and definitely needs improvement. My understanding of IB docs is that a stop should not trigger, using the "last price" method, simply because a trade is reported at or outside the stop trigger price. My understanding of the "last price" method is that the bid (for a buy-stop), or the offer for (for a sell-stop), at the market center where the order is to be sent, must also reach or penetrate the stop trigger price. I believe I was told by one IB employee that IB's stop trigger logic will ignore executions reported at prices more than 0.5% away from the bid-ask spread, which makes sense to me. I was told that IB calls this the "leeway". Another employee gave me a conflicting explanation. He said that the 0.5% leeway means that if a trade is reported at or through your trigger price, AND if the bid-ask spread is within 0.5% of the trigger price, then the stop will trigger. This employee told me that a stop trigger, on the "last price" method, even if the bid-ask spread never reaches the trigger price. I am trying to understand all of this, because today, I had a sell-stop which was triggered by a trade lower than my stop trigger price, even though IB time and sales show that the bid never dipped down to my stop trigger price. It seems to me, based on IB documentation, that my stop should not have triggered, because the bid never dipped down to the trigger price. I suspect that my understanding of the trigger method is the problem, and I hope that somebody will help educate me as to the exact logic used by IB's "last price" stop trigger method.