Dick,
The idea is to prevent the jib sheet from sliding under the mast base and getting pinched or stuck there while sailing (which does tend to happen). On my Prindle, which uses the same setup, I run a bungee cord on each side (personal preference instead of the line) from the flat dolphin striker bar about a foot and a half out from the center post up to where the diamond wire attaches near the base of the mast. When sailing, the jib sheet stops at the bungee cord instead of sliding under the mast base, thus acting as a "jib sheet jam preventer". I must admit, I was puzzled by the manufacturer's manual as well--I thought, "Hey, the guy that sold me the boat left something off!"

Kevin
Prindle 18
Largo, FL