Rope tension cleats like this one have been around for at least over a century. Back in the day a piece of wood or a dowel with two holes drilled in it would suffice.

There is a picture of the tensioner "in action" with a 3/8" nylon rope.

It is very helpful when you are setting up tarps, or tent poles are being staked down (many tents have pre-installed cleats like this already on the ropes). The theory of operation is simple, when you hold the cleat perpendicular to the rope, it moves along the rope freely, when it is at an acute angle, it resists moving.

As you can see from the action shot, you run the rope through the cleat, around the tie down (I use a carabiner) and back to the cleat where you knot one end. To shorten the rope you pull it away from the tie down point, and then release it. To shorten it, reverse that action.

The cleat's ability to hold is intentionally not 100%. If your tarp or rainfly caught a huge wind gust it is usually better that the rope slip a bit and shorten rather than have you tarp or rainfly rip.

This one is printed in PLA, which is easy but it isn't as durable as some filaments. In particular I printed one with Taulman 910 that I expect will last longer the I will! Fortunately it doesn't take much time or filament to print another one.