Why are fencers attached to wires, and is there such a thing as wireless fencing scoring?
The cord is how the scoring machine knows who hit whom. Each fencer plugs their weapon into a body cord that runs up the sleeve and out the back of the jacket, where it clips onto a cable from a spring-loaded reel at the end of the strip. When a tip is pressed, or in sabre when the blade makes contact, the signal travels down the cord to the box, which lights up and beeps. Electric scoring took over from judges watching for hits by eye, which could not keep up with how fast fencing is.
The cord does more than report a hit. In foil and sabre your lamé, the metallic jacket, is part of the circuit too, so the box can tell whether the touch landed on valid target. In foil, a hit off target shows a white light and stops the action without scoring. In épée the whole body counts, and the grounded strip and the bell guard are wired so that hits on the floor or on the opponent's guard do not light up. The box also runs the lockout: once one fencer hits, the other has about 300 ms in foil, 170 ms in sabre and roughly 40 to 50 ms in épée to land a hit as well. After that, their light cannot come on.
Wireless systems do exist, and several companies sell them to clubs. You still plug your weapon into a short body cord, but it ends in a small battery pack worn snugly in a back pocket instead of a reel. They are quick to set up and popular for practice, especially in shared gyms. Results vary by weapon: épée is the easiest to get right, foil can throw false lights, and sabre is the hardest, which is why wireless sabre needs a conductive T-shirt under the lamé. Loose packs and tired batteries are the usual causes of dropped connections.
At tournaments, expect to be on a reel. The USA Fencing and FIE rules name wired apparatus as the standard and allow wireless only for systems authorized by the FIE's equipment commission, and approved systems have only appeared at a few top international events. Unless the organizers announce otherwise, plan on wired boxes and reels. If your club fences wireless, ask for a few sessions on a reel before your first event, because the pull of the cord behind you takes a little getting used to.