Also, taken from bodybuilding.com

"A barbell bench press starting position has the weight lifter lying on a bench, with the shoulder blades pinched together to create a stable, solid base for the press, also used in powerlifting to reduce the range of motion. The lifter keeps his feet flat on the ground or at end of the bench, with the buttocks always in contact with the bench. Powerlifters will arch their back to provide greater stability and to reduce their range of motion allowing them to move more weight. Different grip widths can be used to increase or decrease the range of motion and place more or less emphasis on particular muscles. The movement begins by lifting the bar off the uprights and lowering it until the bar is motionless on the chest before being pressed under control to the start position. After the desired number of repetitions, the weight lifter returns the bar to the uprights. Because the load on the bar above the chest can be heavy, a spotting partner increases the safety of the movement.[1]"