Given what the ancient writers (Proverbs 27:17) knew as “iron”, one could indeed “sharpen” iron implements with other iron. Bill Cravens gives a good explanation. Go to https://www.quora.com/Metallurgy-Does-iron-really-sharpen-iron
Coaching is very much like “iron sharpening iron”. The coach asks powerful questions. This stimulates creativity in the client. Thoughts begin to emerge in the client’s mind based on their knowledge of their experience. The coach may be silent, letting the client filter and sift through those thoughts; letting the client articulate them. The coach person will then ask other questions helping the client to expound more. This creates a sharper focus for the client person.
Amazingly, in as little as 20 concentrated minutes, a person being coached by an experienced, non-directive coach will have a clearer understanding of what they want to do, and a sharper strategy of how to get there.