Andrew.

Your pic didn't load, but I think you are talking about the jib "horse" cable across the tramp. The ends of the cable attach to sliders running in alu tracks rivetted to the inner gunwales.If the cable is not slack, the loading from the jib block will tend to pry the tracks off the hull. If the cable were completely tight/straight across the tramp, the mechanical advantage when a 90 degree force is applied becomes huge. By have what appears as a very slack cable, the force is much better controlled/angled, dispersing the loading at the track.

Fore/aft sheeting angle on the sail is easily achieved by sliding the cable ends back & forth where needed. Inboard/outboard "slot" adjustment is made with the block adjustment lines (green) along the cable. System works quite will, although the horse is a problem while tacking.