Made of a narrow band of spring steel, by careful manipulation, the tape can be “fished” (guided) through the confined spaces within wall cavities. Once guided through, the new wiring can be pulled into the wall by attaching it to the end of the fish tape and pulling the tape back whence it came.
Fish tapes are usually stored coiled on a plastic reel. Because of this, they have a natural curvature and it is this curvature that allows them to be guided. By manipulating the reel, the end of the tape can be directed slightly. The tape is rigid enough that it can then be pushed in the direction in which it is pointing. In this way it can be easily guided through an empty wall cavity. Thermal insulation, firestops, pipes, HVAC ducts, electrical conduits, and other obstructions make use of a fish tape more challenging.
The “tape” can be made from many different materials including steel, fiberglass, and nylon. The tape usually has a special end ranging from a hook or loop to a specialized fastener device to allow the user to attach the tape to the cable before pulling.