If it's the parts I think you mean, then they are tensioners, one internal, the other external. They keep the antenna wires taught, but allow some freedom of movement to avoid undue stress on the anchor points on the airframe. (the fins, and through the canopy frame.) Without these, when travelling at 300 + mph, the movement of the wires, and the effects of 'g', for example in a turn, could cause serious distortion and, at worst, pull free from their anchor points, which could lead to catastrophic damage to the aircraft.