Modern skydiving harness-containers have two containers; one for the main parachute, and another one for the reserve. These containers are built into a single "backpack", with the reserve container above the main. In case the main parachute malfunctions, it is necessary to jettison the main before deploying the reserve to avoid a main-reserve entanglement. This act of jettisoning the main is called a "cut-away".
Over the years, several different devices have been designed for cutting away the main. Among the most popular were variations of the Capewell release system, until in the 1970's, Bill Booth invented the 3-ring release. Variations of the 3-ring release can now be found on practically all sports parachutes.
Cutting away with the 3-ring release is done by pulling a handle placed on the main lift web of the harness. In some cases, parts of the malfunctioning main parachute may be tangled with the skydiver, in which case it may be necessary to use a hook knife to more literally cut away the main.