Some swear it’s magic… the rest of us just see it as a moderately useful exercise for the lower back and glutes.
If you have a reverse hyper machine, use it. If not, you can perform them on a GHD, jerk block, stacked plyo boxes, or similar. Support the trunk and let the legs hang straight down. Ideally the feet can swing under the trunk a bit, but this isn’t always possible. Keep the feet together and swing the legs up as high as possible, keeping the abs tight to prevent excessive lumbar hyperextension. Let the legs swing back down as far as your setup will allow and swing back up. Generally reverse hypers should be done with a bit of momentum carrying through the entire motion. Without a machine, resistance can be added with a light band or a dumbbell between the legs.
The reverse hyper is a glute and lower back exercise that also helps decompress the lower back well and as a result can be helpful for tight backs or low-level back pain.
The reverse hyper can be done light or unweighted as part of trunk prep in a warm-up, or with more volume and intensity at the end of a training session along with other trunk and accessory work. 3-5 sets of 10-30 reps depending on loading is a good prescription.