I think the ARCA trailing stops trigger from the bid/ask quotes and not printed price. For example, if you're long and have a trailing stop order placed, the stop will follow the bid. If you're short, the stop will follow the offer. The reasoning is that this is what you'd have to pay to get out of the position (based on a market order or if you were going to pay the spread). After the triggering of a trailing stop, a market order is placed. Using RealTick (and maybe other programs), you can see the stop price update by having your Order Book up on the screen.
One word of caution with ARCA stops (i.e.- market, limit, trailing): I have had some problems with the stop function working properly but once triggered I've had orders cancelled due to some inapplicable property. I can't recall the details, but I think that one was cancelled because the order type was DayPlus & not Day. Just be cautious until you've used a few.