Explainer-What is the curling double touching controversy about?

CORTINA D'AMPEZZO, Italy, Feb 17 : A controversy over allegations of cheating in curling at the Winter Olympics has made headlines all over the world and plunged the sometimes obscure sport into the mainstream.

Here's what you need to know about double touching:

WHAT IS DOUBLE TOUCHING?

In a legal delivery, a curler must release the stone before the hog line, which lies 21 feet (6.4 m) from the centre of the tee line the curler pushes off from.

A curler can touch the handle of the stone as many times as they like before the hog line, but they cannot touch the granite portion of the stone while it is moving forward.

Doing so results in the stone being 'burned', or removed from play.

World Curling has introduced electronic handles at these Games, which flash green for legal throws and red when players release the stone past the hog line.

Each stone's handle is fitted with a touch sensor, which interacts with a magnetic strip embedded in the ice. The granite, however, does not contain any sensors.

HOW DOES OFFICIATING WORK IN CURLING?

Curling has always prided itself on being an honour-based sport, with players calling their own fouls. World Curling does not use video replays to aid in officiating.

Earlier this month, World Curling board member David Sik, who chairs the governing body's technical commission, told Reuters umpires are rarely called into action.

"There is always an agreement between the skips and teams on how to play the game and how to behave," he added.

WHEN DID THE CONTROVERSY BEGIN?

In a round-robin game at the men's competition on February 13, Olympic champions Sweden accused Canada of cheating by double touching stones, after which there was a heated exchange between Marc Kennedy and Oskar Eriksson.

Sweden's Eriksson