if you mean the rope going through the cleat at the base of the swivelling mainsheet cleat, it is the spinnaker halyard-retrieval line.

The things I like about this system for one up sailing are.

Mainsheet is always in front of you.

Easy cleating, uncleating from anywhere on the boat tramp or trapeze.

It keeps the spinnaker sheet back where it is easily grabbed.

Mounting the spin halyard cleat at the base of the main cleat keeps it close to hand, making cleating and uncleating easy from the back of the tramp.

Regards Gary. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />