This knot is used when tying a shockleader to a mainline. A shockleader is simply a
heavier piece of line tied to a lighter mainline which stops your mainline snapping during casting. If you were using 15lbs
mainline for instance on soft ground where there was little danger of snagging,and used a weight of any more than a couple
of ounces,your 15lbs line would not be able to take the strain and snap. The 2oz weight of course would not let you cast very
far,so in order to get casting distance,you would have to use a 4 to 6 oz weight or so,and you would then need to use
a shockleader. As a general rule of thumb,10lbs line for every 1 oz of weight is used.The shockleader length should be around
3 times the length of the rod so that there are a good few turns of the shockleader around the spool when casting.
Personally I don't use a shockleader when fishing hard ground as I am not casting any
more than 60 or 70 metres and using 35lbs line,and if you get snagged it does create a weak point in the line.
However,when distance casting on beaches I often use a 15ft surf rod with 20lbs line,
4 oz weight, and a 40lbs leader.
So,how to tie the knot.
Lay the two lines against each other as they will be on the rod,overlapping enough to
give you line to work with.
Make a loop along one line with the other in the same way
as the snell knot.
Then take 3 or 4 turns down with the tag end,again,exactly
as in the snelled knot,and pass the tag end through the loop created in stage 1.
Lubricate knot and tighten up.
Then repeat with the other line in the opposite direction.
Then lubricate everything and draw the two knots together
and tighten fully.