It is a depowering technique by increasing the upper spar length above the mast. As the puff hits, the upper spar will bend more allowing the wind to spill off the sail (decrease power).
Need a line (jens line) approximately 10 feet long. You will trim this line to remove the excess tail depending on position of mast cleat.
1. Lower sail.
2. Slide the halyard down (towards the tack) the upper spar. 1/2 to one full clip depending on the conditions. Practice to find out what suits you best.
3. Take the jens line a place a small bowline at one end.
4. Loop the bowline around the top of the mast and over the halyard. Run the tail of the jens line thru the bowline. This will make noose. As you tighten the noose, it will squeeze the halyard against the mast.
5. Run the jens line thru/around the mast cleat.
6. Place a slip loop in the jens line a couple of feet above the cleat.
7. Run the jens line thru this slip loop.
8. Steps 5-7 end result is a 2 to 1 purchase.
9. Tension the jens line and cleat.
My jens line always stretches a little so I always snug it up after sailing for awhile.
Hope this helps...don't hesitate contact me if you need clarification.