Quoting "RW Salnick" SNIP
The light line is very probably the furling line - it wraps around the
drum on the furler. It should be set up so that the line is fully
extended when the sail is furled. When the sail is unfurled, the line
wraps around the drum, thus making it possible to furl the sail again by
pulling on the line.
Snip
I'd add one thing to this -- make sure that the "fully extended" line still
has a couple of turns around the drum after the sheets of the furled sail
have wrapped around the sail two or three times. That will give you a little
of margin of error, reduce the stress on the line's attachment to the drum,
and also help ensure that the furled sail does not come unfurled in a blow
if the wind get's under an edge of the leech. I've seen it happen.
Scott
S/V Itchen