Having been in the powerlifting, bench press, and strongman games for awhile the benching rules states that both of your back shoulder blades and bottom (bum) must remain in contact with the bench at all times throughout the benching process. THe bar must be unracked alone. Must wait until one the judges say "Go!" and the benching proceed. The bar must touch the chest until one the judges says "Go" and then press the barbell with the weights b ack up. And wait for the signal to rack the bar into place. There are judges that watch the bum's contact with the bench. Typically two judges on each side of the shoulders and one judge behind the bencher. And both feet must be flat on the floor at all times during the benching process.
THe arching of the back helps shave off 3 to 5 inches of distance the bar has to travel to the chest allowing heavier weight to bench presss otherwise if the back were completely flat then there would be no way a person could lift that heavy. in my mind the arching of the back is a bit like cheating. It changes the mechanics of the lift by moving a shorter distance.