This was answered well by /u/CChippy a few years ago. In the film an NCO tells the men to have "Nothing up the spout," which means no bullet chambered. This is so that the soldiers do not stop in the middle of No Man's Land to shoot, but instead charge home with the bayonet and take the enemy trenches. Although stopping to shoot might seem wise, the shorter you can get to the other side of No Man's Land the fewer casualties you will take.