Numknuts are described as something very similar to Japanese nunchakus (usually pronounced "nunchucks"), which are a traditional weapon of the Kobudo weapons set and consists of two sticks connected at their ends with a short chain or rope. In the hands of an expert, these are intimidating and potent weapons in hand-to-hand combat. Most people end up hitting their own ear (as did Sir Samuel Vimes).
The word "numb-nuts" is an insult as well, insinuating infertility and impotency, and in relation with the difficulty of using the weapon, probably an apt comparison.
The idea was made to work by the Koreans, who used it to deadly effect in their war with the Japanese (1592-98). The Koreans were prudent enough to put the business end at the far end of a long handle, effectively making the"numknuts" into a pole-arm with a flailing mace on the end, where all the dangerous stuff was happening up to eight feet away, (co-incidentally out of reach of a Samurai's sword) and there was no danger of hitting yourself in the ear with the recoil. The more sophisticated versions had a spring assembly fitted, so that once the flail end had been snapped out to do its lethal business, it automatically recuperated, ready for the next swing.