7 out of 10 people l sail with don’t know that foot lines exist – let alone why !
A Foot line as the name suggests is the tensioning line that runs along the foot of the sail from the tack to the clew and works just like the leech line. In stronger airs you will need more tension than in light airs.
But many sailors /boat owners don’t realise that there is a handy little trim tool that enables you to stop this flapping with an appropriately tensioned leech
It is not really surprising that they can go unnoticed as they are usually set just right when sail is supplied. However, it is useful to be aware of them and know what they do. Eg if the foot of your headsail is fluttering and you are pretty sure you have the sheet trim correct then try tightening the footline tension. Similarly, if the foot of your sail is “curling up” – try loosening the tension.