The draft gives the lee side of the sail an airfoil shape, providing lift and pulling the sailboat along.

PDRacer.com says we should have a draft of about 10% centered about 30% back. You need to add material to both the head and foot to get draft - we shape the foot in the next slide.

It took me some time to digest, then regurgitate these instructions. If they are confusing, look at them again.