Describe the procedures for reducing a sail using a mainsail reefing system and a roller furling jib
Reefing consists of making your sail smaller by partially lowering it. On the mainsail, this is achieved by releasing the mail halyard, which lowers the head of the sail, and by essentially creating a new tack and clew to hold the bottom of the sail down.
Basic layout of the sail
Example of reefing a J/22 boat
Let's look at the reefing system on a J22 sailboat. Our boat uses a jiffy reefing system, also known as slab reefing. This common reefing system works by taking out a "slab" out of a sail.
Conceptually, reefing is very simple. You lower the top of sail and then get rid of the excess at the bottom. Lowering the mainsail is simply done by releasing the main halyard to drop the sail by a couple feet, but how do you know when to stop?
Our sail happens to only have 1 reef on it, which we know by looking at the number of rows of cringles on the sail (i.e. the grommets). For the reefed sail to be functional, you need to secure its new bottom which is achieved by tightening the reef line. The reef line is connected to a the sail cringles (both tack and clew), so when you tighten the line it pulls the what is now the new bottom of the sail down, tensioning the luff and leech.
Going back to our question, you should stop lowering the mainsail once the reefing cringle at the tack is lined up with the boom. At that point, you should secure the main halyard and then tighten the reef line to make the mainsail nice and taught.