In curling, the Free Guard Zone rule is a rule which specifies that stones which stop in the area between the hog line (by which curlers must release the rocks) and the tee line, outside of the house (the rings where points are scored), cannot be knocked out of play until after at least four stones have been played. This rule was instituted in an attempt to prevent curlers from simply knocking one rock after another out of play, resulting in an allegedly dull game.
By the 1980s, curling had benefited immensely from two new developments. First, Canadian ice-makers in particular had developed increasingly fine-tuned ice management, leading to ice surfaces which were more uniform than ever - and therefore allowed curlers more control than ever over their rocks. At the same time, new types of curling brooms, or brushes, also allowed increased control over the path of the rock through sweeping. This led to the widespread adoption of a strategy of knocking all stones out of play, referred to as "peeling" stones. It was relatively easy for skilled curlers to knock opponents' stones out of play, by striking them at angles which also caused their own stone to leave the ice. The team with the hammer - that is, the team going last - would then have a relatively easy chance to score one point.
Normally, skilled curlers attempt to construct intricate arrangements of stones, with "guards" placed in front of the house to prevent their opponents from easily knocking their scoring stones out of place. Peeling stones off one by one eliminated this strategic element to the game unless and until the team with the hammer happened to miss one of their shots. It was a solid strategy - but it made for boring and simple games, more like bowling on ice than traditional curling.
The Free Guard Zone rule was created to return curling to its more traditional format. The first Canadian rules specified that the first three rocks on the ice could not knock an opponent's stone out of play and then leave the ice themselves. This was subsequently expanded into the international four-rock Free Guard Zone rule (which the Canadian Curling Association adopted years afterward in place of its own). The new rule means that curlers cannot play the old elimination game until after at least four stones are on the ice - which is usually enough to allow strategy to re-enter the game. If a guard rock is knocked out of play improperly, then it is returned to its original position after the shot rock has left the playing surface.
Very highly skilled curlers at the international level, such as the rink of 2010 Olympic gold medal winner Kevin Martin of Canada, have since devised a method of sidestepping the Free Guard Zone rule through a technicality. The rules specify that a guard rock cannot be knocked out of play, but not that it cannot be nudged and moved while remaining on the playing surface. The new strategy is therefore to nudge, or "tick," the rock, so that it is still in play but is now out of position and therefore useless as a guard rock. The shot rock then slides out of play, as before. However, ticking is vastly more difficult than peeling - and increases the risk that, if the guard rock is knocked out of play, it will be replaced anyways. For this reason, it has become one of numerous strategies available to very highly skilled curlers, but not a dominant feature of the game as peeling once was.