J:son Ultimate Anti Twist Fly Swivel

Swivel installation on leader: Weight the butt end of your leader down to the top of a table or bench, with the thin end hanging down over the side.
Thread the swivel to the tip of the leader, loop up and pin down, and then “balance” your leader and swivel as follows:
Move the swivel gradually upwards while cutting the leader behind it until the leader stops bending
  

downwards from its own weight. When you can hold the leader between your thumb and forefinger with about 2 in / 5 cm of leader and swivel remaining straight (horizontally), without bending or the swivel sliding back, you have found the “sweet spot”. Remember to leave a few extra inches of leader material for the knot when you attach the swivel.

Attach the swivel using a Loop, clinch or grinner knot, cutting off the tag end as closely as possible. Also, straighten out the knot by pulling the swivel so that it hangs perfectly straight as an extension of your leader. Next, tie on your tippet (terminal part of the leader), about 4,9 – 8,2 ft /1,5 – 2,5 m long, depending on where you fish and what size of tippet you use. I normally use about 4,9 – 8,2 ft / 1,5 – 2,5 m of 0.10 – 0.14 tippet to avoid drag in moving water with wary, selective fish, and about 3,3 – 4,9 ft / 1 – 1,5 m of 0.16 – 0.20 tippet for larger waters requiring longer casts and lots of “false casting”. You have now balanced your leader and swivel. If you have done it right, you’ll be able to cast a helicopter into a headwind without your tippet getting twisted or tangled, and without the swivel affecting your presentation, no
matter what fly you’re fishing with.