If the ball goes out of bounds in say, a soccer game, it’s up to the referees to call the violation. But in a high-level curling match, the stones are somewhat self-policing—thanks to the red and green lights atop each one.
As long as the curler releases the stone before the front of it crosses the first red line (the “hog line”), the lights will stay green, and play will continue. If you see the red lights come on, that means the curler kept their hand on the handle too long, committing a hog line violation. The shot is disqualified, and the stone is quickly removed so it doesn’t interfere with other stones on the curling sheet.
The detection system, called “Eye on the Hog,” dates back to the late 1990s, when University of Saskatchewan engineering professor Eric Salt suggested that some of his advanced electrical engineering students invent one for a class project. With his help, they did, and Canadian engineering company Startco modified their design and brought it to market. The Canadian Curling Association soon adopted it for official use, and its popularity grew from there.
Basically, there’s a magnetic strip installed beneath the ice slightly behind the hog line—or, more specifically, the length of one stone’s radius before the edge of the hog line [PDF]. Inside the stone is an electrical circuit whose current is affected when you’re touching the handle. If you’re still touching the handle when the stone crosses the in-ice magnet, it’ll set off the battery-operated red LEDs. Release the handle before the stone reaches the magnet and the green lights will flash until the stone gets there [PDF]. At that moment, the flashing will switch to a steady green to indicate that the release was valid.
This system also explains why the player releasing (“delivering”) the stone doesn’t wear a glove on their delivery hand: They’re not allowed to, since gloves inhibit the touch sensor [PDF].
Due to the high caliber of Olympic curlers, hog line violations aren’t common in Olympic curling matches—so you don’t often get to see the red lights in action. But they came in handy when Italy’s Stefania Constantini kept her hand on the handle just a bit too long during a mixed doubles match against Norway on February 3. (Italy still won.)