Brebebe Door Anchor Strap for Resistance Bands Exercises, Multi Point Anchor Gym Attachment for Home Fitness, Portable Door Band Resistance Workout Equipment (1 Door Anchor+Resistance Bands Set)

It works like a moving strap or tie down harness. You put it around the door and tighten it up. It then has an assortment of loops to either put your resistance band through or hook up a carabiner to then lock to your bands that have metal loops. I found that the top ring and the bottom ring work best. Anything in between and the strap just won't hold the tension. I sent mine back for that reason. The whole point for these door straps it to maintain tension so you working the band not fighting the slack from the door.