The “five-rock free guard zone” is simply an alteration to the previous “four-rock free guard zone” rule.
The free guard zone is relatively new in curling. Back in the day, teams could take out guard rocks as soon as they were placed. This led to a clutter-free game, but also low-scoring, predictable outcomes that weren’t very interesting for spectators.
In the 1980s, Russ Howard came up with the “Howard Rule” based on a practice drill his team used where the first four rocks in play could not be removed at any point during an end regardless of where they were placed. This kept more rocks in play and made for a more interesting game. It was used at an event in Moncton, which is why it’s also referred to as the “Moncton Rule”, and was a hit with the players.
A modified version of this was adopted as the four-rock free guard zone by the World Curling Federation for the 1991-92 season. In this rule, a takeout cannot be played on any stone sitting outside the house from the tee line up to the nearest hog line (the “free guard zone”) until four rocks have been played.
Canada adopted a three-rock rule before moving to four in 2002.
However, teams began to find ways to work around the four-rock rule by “ticking” guards aside, which pushed them out of the way but didn’t actually eliminate them.
The Pinty’s Grand Slam of Curling was the first major organization to experiment with a five-rock free guard zone in December 2011 at the Canadian Open. An extra guard in play can lead to more aggressive play and give more control to the team holding the hammer as now the sixth rock in play can dictate the flow of the end.
The Pinty’s Grand Slam of Curling implemented the five-rock rule for all of its events following a players’ summit in the summer of 2014. The World Curling Federation followed suit ahead of the 2018-19 season.