You want a low stretch pennant.

Here's something interesting

http://www.atninc.com/tacker.html

Infor on the sail and how it is used.

http://na.northsails.com/Cruising_Sails/gennakers.html

Martin Baxter wrote

The boat came with a bag containing a sail marked "Genaker", so I took
the sail out, it seemed to made out of about 1/4 oz cloth, radial on the
top, parallel panels on the bottom, upper corner marked "Head", one
lower corner marked "Clew", other marked "Tack". The tack corner has
about 30 ft of 1/4" nylon cord spliced to it and a two foot length of of
1/2" nylon tape with a snap shackle leading from the same cringle.

I have never used such a sail, I would deeply appreciate it if some kind
soul could tell me the ins and outs of flying such a sail.

I attached the "tack" to my stem fitting with the nylon pendant(?) and
used a piece of 1/4" dacron as a sheet from the clew, the thing seemed
to fly OK, but the wind was so light and fluky that it was somewhat
difficult to tell if I was doing this the right way.

Why is there a such a long piece of line spliced to cringle marked
"tack"?

Cheers
Marty