For those of us that do not have headsail roller reefing, it is necessary to make sail changes according to requirements. If you are single handed this becomes a difficult operation, trying to hoist the sail and feed it into the luff track at the same time.
I first saw this simple gadget in a French chandlers made from plastic and costing about 100 FF (£10). This seemed a bit pricey for a piece of plastic, so I have made one from 10mm ply. The picture explains how to make it and it is simply tied to a convenient place at the foot of the forestay. Once the sail luff rope is fed into it, one is free to hoist the halyard and the feed will take care of itself.