The Hyperextensions, also known as back extensions, is an exercise for the posterior chain, the group of muscles running on the backside of the body. It is primarily targets the glutes and erector spinae, which is responsible for extending your spine. And it will also work the abs, core and hamstrings as secondary muscles. If you choose to hold a weight plate, it will also work your arms as stabilizers.
Strengthening your lower back will improve posture and help to reduce and prevent lower back pain. It is especially important if you sit infront of a desk most of the day. The Hyperextensions will also help you to get better performing other movements that require a strong lower back, such as the deadlift or squats.
Hyperextensions are usually done on a dedicated bench, but if you don't have one you can also do Hyperextensions on a regular flat bench. Although being less comfortable, it will work your muscles in almost the same way. With the biggest difference is that the range of motion will be shorter due to the height of the flat bench.
The Hyperextensions can be done as a body weight exercise. But you can also add resistance if it gets too easy. Just hold a weight plate or a kettlebell near your chest.
To keep your balance tuck your feet under something steady, or ask someone to hold your feet down.
Move in a slow and controlled manner. It is very important to control the movement. Don't just let your torso drop down, but lower it slowly instead. Swinging the torso too fast might result in a bad form and will increase the risk of injury, especially if you're holding extra weight.
Keep your back straight throughout the whole exercise. Do not round the back, and don't over-arch it at the top position, go up until it is on the same line as your legs. Allow a neutral slight arch in the lower back.
Keep the neck on the same line as your back. This will keep your neck safe and prevent putting too much pressure on it.
Only go down as low as you can while still keeping a good form. Everyone's body is a little different, how much you can bend is depended on you flexibility and body structure. Try to be familiar with your limits and make sure to keep a good form throughout.