SM
You are probably a very competent OJTI. Many controllers will be without any course.
Historically little training was given for this role. I can remember being trained by one person who had no concept of transferring skills and sat perched on the edge of their seat saying what I should say to EVERY RT call.. It became a contest for me to beat him...
Later in life I was interviewing a very competent OJTI as to the skills required to teach on the job. He, as you have said - stated none, you just know what to do. He did have 35 years experience and was a natural teacher.
A good OJTI course will give you a good insight into how your trainee is learning and some techniques you can use to help.
I have run a number of courses, for various organisations, over the past years and ALL the students have agreed that they learnt useful concepts and techniques. Not all these will be used, but many will help both the trainee and the Instructor.
OJTI courses have not been designed just to please regulation. They are considered necessary to give the required skills to ALL OJTI. Not just those with natural ability.