This is a pretty simple exercise that I see done wrong a good portion of the time. First, it’s a back “extension”, not a back “hyperextension”. There’s no need to force your lumbar spine into hyperextension. I don’t like it and neither does your lower back. In the video below, you’ll see my client getting too much hyperextension early on in the set, but as I coach him up, there’s less hyperextenison. Second, the speed at which I see some people do this exercise is ridiculous. You’re not bobbing for apples here. You’re trying to strengthen your lower back. Use a nice controlled tempo. The third thing I tend to see a lot is too much rounding of the upper back. Set the pad adjusted to the right height, so that you have freedom to move and can hinge at the hips. Then try to keep a relatively neutral spine throughout the movement.
Another point to make here is hand placement. I’d usually start a client off with hands placed across their chest, progress to hands behind their head, may be progress to hands overhead at arms length and then add in some external load by having them hold a weight across their chest. Don’t hold the weight behind your head!!