The NYSE website states that, "8:30 am - 9:00 am Imbalance and Paired-off information is disseminated every 5 minutes for each security 9:00 am - 9:20 am Imbalance and Paired-off information is disseminated" My question is that if order imbalances are seen at 8:30am, what is to stop the market from getting manipulated by large opening orders being placed and then cancelled before 9:28am?
It's illegal to manipulate markets. If the SEC doesn't get you, NYSE will hunt you down and break your legs. Large orders come from large players and they know better than screw with NYSE. Keep in mind that NYSE stocks do not have to open at 9:30 so there's lots of discretion.