If you have watched curling before, you may have seen to green lights illuminate near the corners of the stone as it is released. These lights are connected to a sensor that can tell when a curler releases a stone, and is a part of the electronic hog line device. The purpose of these lights is to make sure that the thrower does not commit a Hog Line Violation.
A Hog Line Violation occurs when a curler fails to release the stone by the time the front edge of the stone reaches the hog line. If a Hog Line Violation is committed, the stone is immediately removed from play before it even reaches the other side of the ice. This is so that it does not disturb any stones that are in-play on the other side of the ice.
For local curling clubs, stones will not contain the sensors that can tell whether a Hog Line Violation was committed. Therefore, it is expected that curlers be honest if they do commit a Hog Line Violation. In nationally/internationally competitive play, stones will have the sensors that can tell whether a Hog Line Violation was committed. If the green lights illuminate, a Hog Line Violation was not committed, and the shot continues. If the red lights illuminate, a Hog Line Violation was committed, and the stone is immediately removed from play.
If any part of the stone is over the hog line before the stone has been released, it is a hog line violation. Even if a small sliver of the stone is over the hog line, and the stone has not been released yet it will be called. If a hog line violation is detected, the shot cannot continue, and the stone is immediately removed from play.