Chris,

I asked the same question earlier this year - it was on Frank's I20 my F18 as well. Apparently, before they started building the mainsails with the plastic sail slug on the luff/foot of the main, it was possible to pull the main sail out of the mast track by cranking the outhaul. The line was put in place to limit the outhaul travel so this wouldn't happen. I use it now on my F18 but only set it to my desired upwind setting so that when we're rounding Cmark, I just yank on the outhaul until the line catches it. I don't have to look at it to know it's where I want it.