Sergeant First Class Clifford L. Roberts, U.S. Army, Special Forces, drew up the first design on a napkin, after a wounded Special Forces Soldier fell out of a McGuire extraction rig, during a combat extraction mission.
The operator was extracted using a dual rope (or strap) 'Y' design system (one per each STABO rig), lowered by a helicopter.
Once both carabiners were attached to the upper V/D rings on the rig, the operator could then be lifted out vertically by the helicopter.
As the STABO rig was used as the base for operator's personal LBE harness, it was worn for the full duration of the combat operation, in which to allow for rapid extraction by the rope ("strings") method, if a conventional helicopter LZ could not be quickly established (which was frequently experienced in South East Asia, as reconnaissance teams were operating deep in heavily forested enemy territory).
The current US Military method for extracting troops using the helicopter rope method is the Special Patrol Insertion/Extraction (SPIE) system, a direct and close descendant of the STABO rig system that was developed and pioneered in Vietnam/SEA.