The typical IDW helps to pepare potential instructors to teach a skills class in a discipline. In general, instructor candidates need to be pretty solid in their chosen field before attending an IDW as there really isn't enough time to develop all of the skills. It is highly recommended that instructor candidates take a formal skills class before attending an IDW and take time practicing the necessary skills. That said, nearly all candidates have a skill or two that needs some refinement and an IDW class is perfect for that.
IDW classes are more than skills development though. Our goal is to produce solid instructors. It is certainly possible for an all-star athelete to fail the overall instructor certification. Instructor candidates will learn a great deal of soft skills like presentation, time management, learning theory, planning, etc. An IDW will also teach candidates a geat deal about the ACA itself, history, resources, and of course paperwork.
Many trainers also incorporate a skills assessment as part of an IDW so you may be awarded a skills assessment at the end of an IDW.
The IDW is just the first step in becoming an ACA certified instructor.