MacGregorSailors.com

I found and excellent video on YouTube that addresses the use of the backstay, what rigs it works on (spoiler alert! Fractional), and when to apply it.

I thought I had it figured out, but I was wrong. Check out the video:

https://www.youtube.com/watch?v=9GDro9i1ye0

When you bend the mast, the middle section moves forward more than the top moves back. That pulls the middle of the sail forward, flattening it.

--john

That's it in a nutshell. The important caveat here is "on a fractional rig". On masthead rig of course all it will do is tighten the forestay and bring then top of the mast slightly toward the stern.