The stock of Iridex, a maker of lasers used to treat visual ailments, had been hovering around $3.43 all day on June 29. At 3:55 p.m., five minutes before the market close, it took off.

It moved to $3.65, then $3.80. Less than a half second before the trading day and calendar quarter ended, Iridex jumped 4% to $4.17, capping a nearly 22% rise for the day.