Island allows to post hidden (non-displayed, "iceberg") orders that do not appear on the limit order book. I can see a large amount of those executed for QQQ, for example (apparently somebody using them a lot). Do these orders provide any real advantage for traders and if so, in what situations? Thanks,
yes, there can be a big advantage - same way the institutions use the reserve book for redi/arca and inca. point is - you don't have to show your hand.
All ISLD orders for QQQ,SPY,DIA,SMH are hidden thanks to the SEC. http://www.island.com/prodserv/developers/resources/emailarchive/20020919.asp