If you have been doing the same thing for many years, training someone to do your job can be quite difficult. Numerous factors play into this training challenge, but the concept of being unconsciously skilled plays a huge role in this challenge. What does unconsciously skilled mean? Basically, you have been doing something for so long, you can do it without being aware of your actions.
Think of the first time you tried to ride a bicycle. You struggled to remember what to do, how to balance yourself, stop, etc. Once you master the art of riding a bike, you merely get on and ride away. You no longer have to think about how to do it. You just do it.