Like for coaching, the why-how approach does not require the facilitator to be an expert in the field that will be mapped. The facilitator should be knowledgeable about the questions to ask and when to ask them, in order to document the desired why-how map, as described below.
The person who answers the questions should be the subject matter expert, proficient enough with the objectives, the options, as well as the benefits for self and for others.
Caveat: The why-how coaching approach described here is a simplified version of coaching techniques that provide multiple powerful tools. Ref. International Coaching Federation (ICF).