A Grigri (or GriGri, Gri-gri or Gris-gris) is an assisted braking belay device manufactured by Petzl designed to help secure rock-climbing, rappelling, and rope-acrobatic activities. Its main characteristic is a clutch that assists in braking under a shock load. The success of this device has led to grigri becoming a common name for devices of this type. In 2011 a new version, the GriGri 2 was released to replace the original which has been in production since 1991. Competitors include the Faders Sum, Trango Cinch and Edelrid Eddy.
Grigri (climbing) Wikipedia
The Grigri works by pinching the rope when it is moving quickly (like in a fall), making it an assisted braking belay device unlike traditional belay devices such as a Sticht plate or an ATC. Internally, the rope runs along a cam, which allows the rope to pass if moving slowly but when the rope moves quickly the cam will rotate, pinching the rope.
Petzl recommends the device for lead and top-rope climbing.
When used correctly, the Grigri's camming mechanism can assist in holding a climber that is working a route, or hanging on the rope while trying to figure out a climb. When belaying, the same technique for "taking in" that is used with an ATC or similar device is used. However while paying slack out into the system if the device is held open (with one technique being referred to as "the thumb") and the climber falls, unless the belayer lets go of the gri gri's brake mechanism the device will not lock. The device is also more likely to lock if the belayer is holding the brake rope, although any descending brake rope is likely to cause the device to lock, whether held or not.
The Grigri has a lower limit for the rope size for which the cam will engage; the manufacturer recommends the Grigri 2 be used only with 8.9 to 11mm diameter ropes. This makes it unusable with thinner ropes like those used in many alpine applications.
This device has just one place for installing rope and it can't be used in climbing with half rope.
While designed as a belay device, big wall climbers have invented novel ways to use the Grigri that are not recommended by the manufacturer. For example, some big wall soloists use the Grigri (sometimes slightly modified but not necessarily, as modification is not recommended by the manufacturer) as a self-feeding hands-free self-belay device. In big wall situations, the Grigri allows for hands-free belaying on long aid pitches while the rest of the party does other things. It can also be used by the second to self-belay while jumaring the rope as one half of the ascender pair; the leader can belay the second hands-free allowing the leader to haul, take pictures, or do other chores; the second can use it to lower out while following a traverse.