At my broker sell stops are activated off the bid. When selling begins and the actual price begins to drop, what it means to me is that bids are being filled. When I see an order filled at say $20.50, then $20.40, then $20.30 it means that the last best bid was $20.30 but my screen is still telling me that the best bid is $20.50. If I have a stop order at my broker at $20.45, and since the data on my screen is supplied by my broker, I know that the order hasn't executed. What this tells me is that bid prices are not updated promptly. Why? I don't know. Why are sell stops activated off the bid price? I don't know. Maybe it has something to do with maintaining an orderly market. If the actual price can be updated instantaneously then I know that the best bid price can be updated instantaneously too, but for some reason it isn't. Why? I don't know. But I sure as hell am not going to depend on stop orders. I think stop orders are better suited to investors and not traders.