This is by far and away the very best connection between a light main reel line (12lbs to 25lbs) and a heavier casting shock leader (50lbs to 80lbs).
First, form a simple overhand or granny knot in the end of the shock leader. Now pass the main line through the open granny knot and form a large loop of main line at the rear of the leader. Pass the free end of the main line over the leader line three times and bring it back towards the rear of the loop. Pass the free end of the main line round the leader line inside the original loop. Tighten the granny knot first, then slowly tighten up the main reel line knot. Clip off both tag ends very close after fully pulling the knot together.
This gives a tapered at the front streamlined knot that passes smoothly through rod rings during power casting, but it can catch a little against the rings under heavy fish pressure when retrieving. Strength ratio is in the low 90% band.
Rod n Reel Combos
