Carrying a revolver with an empty chamber under the hammer harkens back to the days of the Colt Peacemaker and similar revolvers. Those guns had the firing pin on the hammer, a long kind of triangular extension of the hammer that went through a hole in the gun frame and into the chamber of the cylinder that was under the hammer. A good bump or a dropped gun could result in the gun going off. Due to the kind of activities cowboys and others who worked outdoors did, an unwanted discharge of the gun was a very real possibility. That was their safety and it didn't have to be taken off to shoot the gun, they just had to cock it to get a round under the hammer. Of course if they knew they were going to be using the gun for self defense they just flipped open the cylinder gate and stuck in another round and were fully loaded. That had noting to do with not being trained in the use of their weapon, in fact it was just the opposite, it was smart. Once the transfer bar was invented and became common place it was no longer necessary to carry with the hammer down on an empty chamber.