Intermarket sweep orders (ISO) are limit orders that require they be executed in one specific market center even if another market center is publishing a better quote[1], disobeying the order-protection rule. They are typically used by institutional algorithmic investors and not by individual investors.[2] Intermarket sweep orders were thought to have played a role in the May 6, 2010 Flash Crash, but upon further examination it became evident that it was market orders and not ISO orders that caused the crash[3], as by definition an ISO has a price component and cannot cause a precipitous drop in price.
This isn't correct. Intermarket sweep orders sweep several different market centers and scoop up as many shares as possible from them all. These work against the order-protection rule. For example, if a trader is trying to buy 100 shares of X, and there are 10 shares of X being offered at $1 at one exchange and 1000 at $1.10 at another exchange, the order protection rule would let you buy ONLY those 10 shares at $1, after which you would need to send in other orders. With the ISO, you can buy the 10 shares at $1 and the remaining 90 at $1.10 on the other exchange subsequently.