This weekend marks the two-year anniversary of the aptly named flash crash. On May 6, 2010, starting at 2:42 in the afternoon, the Dow Jones Industrial Average plummeted nearly 600 points in just five minutes, only to recover most of that loss two minutes later. A few large U.S. companies witnessed their stocks trading for as little as a penny a share before snapping back to near where they had been trading when the day began. What would be the second-biggest trading day for U.S. equity markets — 19.3 billion shares crossed the tape — left investors, regulators and most journalists scratching their heads.

At the....