They do just what it says, a heavy-test fishing line with the odd, but convenient shaped hook on the end comes out to about 5 or 6 feet, and retracts when you push the button on the side just like a YO-YO. It will lock in place at a desired length, or you can just leave the tension on to support the tops of your plants. The tension is set at a perfect torque so no stress or damage is done to your plant.
Next time your at the hydro shop and you have a few bucks extra to spend, I think you will find constant convenience with a few of these hanging above your plants. Be well....