A basic principle of U.S. stock exchanges is that the first investor to place an order at the best current price generally should be the one whose order is filled first.

But critics say high-frequency traders can jump ahead in line via special order types, like "Hide Not Slide." Here's how it works.

Say an order to buy Microsoft Corp. for up to $30.01 a share is sent to electronic stock exchange Direct Edge Holdings LLC, with instructions to be filled only there and not routed elsewhere.

Meanwhile, though there is no matching sell order on Direct Edge, another market, such as Nasdaq, has an order to sell Microsoft at $30.01. It is also an order to be filled only on that exchange.

The SEC considers this a "locked market" and doesn't allow it. The fear is it could encourage manipulation such as buying and selling a stock merely to generate fees. The ban means an order to buy for $30.01 can't be displayed on Direct Edge. The order will "slide" to a lower price, $30.