I dont have a trailer, I keep mine parked in the boatyard but I'm pretty sure I know what that is for anyway. Normally when you step the mast using the metal bracket Clint was talking about, you have one guy tie a rope onto the front stay and disconnected it and then start to lower the mast backwards. but once it gets down at a certain angle, because the rope is now so low to the mast this guy is at a major mechanical disadvantage and wont be able to hold the mast from comming crashing down (which is bad). S normally you have another guy standing in the trampoline and then the first guy loses his mechanical advantage, the seconds guy just grabs the mast and holds it up from there and lowers it slowly. If you are by yourself though, this is not an option. This device extends the effective point thatyou are pulling from out so that you never lose that mechanical advantage and the guy on the rope tied to the calbe stay (you) can lower the mast all the way down before the mechanical advantage is lost. Does this make sense? Try to think of it in terms of torques and effective perpendicular distances.

on a side note, if you are transporting the boat on the trailer then yeah you have to step the mast of course, but if you are just doing repairs, like say you lost a rope and it pulled all the way to the top, it is much easier just to pull the boat into an open area and flip it down on it's side. This can be done alone also. First you just muscle one of the hulls up and get the boat ballanced at an angle so you arent having to hold onto it too hard in either direction. Then while carefully keeping it balanced move under that hull, and step over the other one. As you step over the lower hull set the center of balance off a little so the boat starts to fall on it's side. It's ok now though because you are right there underneath the mast holding onto it so you can lower it carefully. It sounds kind complicated the way I described it but it's really pretty intuitive. Just be carefully not to let the boat come crashing down in either direction.