I'm looking at some highly traded NASDAQ stocks and notice the bid/ask changing alongside the direction of the ES (or NQ), like I see the ES go up and see the bid grow larger slowly, etc. I want to answer my own question and just say they're reserving an order there for the off-chance someone hits the price out of line with the futures, but wouldn't it be more likely that when these guys who join at the back of the queue get hit that they are getting hit because of some crazy bad news? I almost never see the queues getting cleared against the grain of the market unless there is news or some shock. Are these guys just arbitrageurs looking to reserve position in the off chance they get taken out when the spread is favorable to them, or just holding a spot for when they need an out later?