Rod Harrison, who is a great friend and a top fly fisherman in Australia, frequently gets in fights with big billfish, tuna and other nasty critters. He is a strong advocate of gel-spun line for backing on fly reels and has developed a method that eliminates the problem. First, he has found that when you build a Bimini Twist loop in gel-spun line, you shouldn't make the normal 20 turns we associate with Dacron. Instead, make 40 or 50 twists before finishing the knot. Once you have this loop, don't loop it over the fly line once. Instead, repeat this at least six or eight times. When this is pulled snug, it will not slip.