A trysail is a small storm sail that is flown in place of your mainsail. It usually lives deep in a sail locker stuffed into a bag. Here it typically remains for the entire life of your yacht.
The idea behind a trysail is that should the winds pick up to severe speeds, you can drop your mainsail and raise the trysail. This takes all the stress off your mainsail and puts it solely on a dedicated and purpose built sail.
The problem in this narrative is that the sail is stuffed away where it is forgotten, and therefore, seldom used. In a storm, the last thing you want to do is root through a locker, pull out a sail, remove the mainsail, attach it to the luff, and then raise it. When a storm hits, you want to make your main smaller and get back to the cockpit as quickly as you can!
Having a dedicated track for the trysail allows you to set it up before you leave port so that should the situation arise, it is ready to go.
We flake and then ball up our trysail at the foot of the mast, where it remains laying in wait for the moment we might need it. We have the starboard sheet tied to the clew, so all we need to do is attach the port sheet once the mainsail has been lowered; and its dedicated halyard already attached.
When we need the sail in a hurry, we just drop the main and raise the trysail. The starboard sheet is set, so worst case scenario where we don’t have time to attach the port sheet before raising, we can sail on port tack. The clew of the trysail is low enough that I can easily reach it to tie on the port sheet after it has been raised without reaching overboard or far off the deck (I’m tall though).
With the trysail setup like this, we find that we use it very often, which makes our blue water passages very relaxing and safe, since we can don the storm sails just as easily as we could raise our working sails.