What is the truth about arching your back while bench pressing?

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When I was a young teenager in my high school fitness class, we were always told not to arch our backs while benching because that was improper form. Up until recently, I have always understood that to be the case. then not long ago i was reading some youtube comments where people were saying that you are using bad form if you are *not* arching your back. what is the truth? source will be necessary for best answer.