The back should be arched to some degree. Upper and all of Back should be activated. Feet planted on the ground. If you ever do any heavy weight, your back is going to be working anyways. Ass will be planted. Tripod on the bench with two shoulder blades and one ass locked. Powerlifters super arch to decrease distance and rom. Just like you'll never see a powerlifter going for a PR with his feet on the bench or in the air ala "getting ready to crunch" style...(whoever thought this shit up)
We talked alot about the role of the back in bench pressing. Find a powerlifter that doesn't have a wide strong back. find ANY power move or power position in the gym or in real life, such as yard work, where the lordotic curve isn't the base.... All in all, it doesn't matter as far as the minute details go as long as you have your foundation down. A powerlifter will be able to decrease arch, do a "normal" bench and still bench their ass off because they've been lifting heavy weight! Cheating? Not really an issue. the pecs aren't doing much. Contracting, and relaxing. A good rotation and press and it's all over. To me the most important points I see totally wrecked is first and foremost not rotating through the RC: Not bringing the elbows in and thus "shouldering" up and down the weight. Once you've done that unsupervised since weak ass gym/football classes and you've got thousands of reps and a motor engram that is hard to overcome. ... Main problem with those that can't grow their chest or have shoulder issues. Then the dancing feet syndrome. No support is no power. Feet dancing all over the place. Not to be a meat head, but benching is a full body exercise or should be. And more and more lately I've been warming up with like 85, sometimes less, for a few sets. All of my warmups are strict, not going through the motions. And super tight grip on the bar for the heavy lifts makes a big difference in stability and cns activation.
Of course when you're getting older, you really start to pay attention to the little stuff.