Do you ever see videos of people benching, and wonder to yourself why they're creating a big arch through their back? Surely that's dangerous, right?
Actually, it's not dangerous at all, and powerlifters and strength trainees have been using this technique safely for a long time now.
Why It's Safe
When you are benching, the weight of the bar is only acting on the muscles and bones that are directly beneath it. This means that everything from your mid-back and down is not taking any of the weight. All of the weight is acting on the upper back, which is locked firmly in place on the bench pad.
This is very different to a squat, where the bar is pushing down directly on your entire body, so you must maintain a neutral spine.
Should You Use It?
There's a common misconception that only powerlifters need to arch. In reality, everybody can benefit from using the arch, as it not only allows you to move more weight, but it's also safer for your shoulders.
One of the reasons that a bench arch is used in powerlifting is to shorten the distance that the bar has to travel, as your touching point becomes higher. There is a debate about whether it's better for a non-powerlifter to sacrifice moving more weight in order to use a greater range of motion. I would agree here in the case of a person with a highly flexible back. If you look like the guy below and you're training just for general strength and muscle gain, then I'd recommend reducing the height of the arch.
However I personally believe the greatest benefit of arching has more to do with stability and shoulder health. What newbies to benching do not realise is that pushing heavy weight requires a strong upper back, and the ability to keep your scapulae (shoulder blades) wedged back and down.
If your back is flat on the bench, you are in a much less stable position and you'll roll your shoulders forward as you press, which is the main contributor to shoulder pain in the bench.
Arching allows you to lean more of your body weight onto your upper back, allowing you to keep your shoulder pulled back in a safe and stable position. If you are unsure about how this works, just do a glute bridge and feel what happens to the pressure in your upper back - it's the same effect as an arch.
Arching in the bench is the safest and most efficient thing you can do with your back. If you're a powerlifter, try to get your arch as high as you can. If you're not, then arch just enough so that you can keep your shoulder blades in the right position and keep your upper back tight.