I didn't get how this could work until I started sketching it out.
One of the things that absolutely requires two agile, strong people on a sailboat is raising the mast. Even at the best of times: on-the-hard, with no wind and two able crew, it can be a harrowing, often risky if not dangerous exercise as the mast wobbles side-to-side.
So when someone on a pocket-sailer board came up with an idea that might just make it loads easier, I paid attention.
It is nothing more than two poles (broomstick handles) that are attached to the cabin roof and joined at their tops to a slug that slides through the slot in the mast. As the mast is raised, the slug slides through the slot. The rigid poles constrain the mast from sliding to either side as it is raised.
I love its simplicity. Simplicity is one of the Golden Rules on a sailboat. At least, one of the Golden Rules on my boat.
Also, I like drawing pictures.