The caption for the image ISS016-E-008937 says
ISS016-E-008937 (3 Nov. 2007) --- While anchored to a foot restraint on the end of the Orbiter Boom Sensor System (OBSS), astronaut Scott Parazynski, STS-120 mission specialist, assesses his repair work as the solar array is fully deployed during the mission's fourth session of extravehicular activity (EVA) while Space Shuttle Discovery is docked with the International Space Station. During the 7-hour, 19-minute spacewalk, Parazynski cut a snagged wire and installed homemade stabilizers designed to strengthen the damaged solar array's structure and stability in the vicinity of the damage. Astronaut Doug Wheelock (out of frame), mission specialist, assisted from the truss by keeping an eye on the distance between Parazynski and the array.
It can also be found in Wikipedia.
Is this cable simply an emergency backup if something breaks, or is one potential use to allow the astronaut to disconnect and move farther from the end of the boom. If so, has it ever been used like that?
Full size (1.7 MB) if clicked: