It goes back to being "one with the locomotive". Some people have the touch and let the engine tell them what is happening - or about to happen - others are without a clue. I remember the engines making a bit of shudder before starting to break loose. If you recognized that and put down some sand, the engine might take and extra step or two before catching herself and settling down. I can remember firing for someone who didn't have the feel. I'd be over there watching the water or the steam gauge, when I'd feel the shudder, I'd quickly look over at the engineer, who was gazing out the window, oblivious to the signals the engine was sending to him. I'd open my mouth to say something when the engine would take about 3 extra steps, the completely spin out. By the time he woke up, got the throttle closed, turned on the sand and opened back up we had lost noticeable speed, and my fire was pretty well sucked up the stack.