The info is available on the Pro Site. Just go into Your Account and go to the link for Online Specialty/MSDT Instructor Application. There you will find the Specialties you can apply for based on the "20 dive" rule. I have only done one specialty that way - I felt I had learned enough tips and tricks through assisting other instructors to warrant simply applying for the teaching certification.
For the other Specialties I can teach, I received the teaching certification after instruction by a CD. I learned a ton of stuff that way - teaching tips, tricks, etc. that were invaluable. There would be no way I'd attempt to teach those without the coaching - I am still quite new to teaching, so the help was what I needed. My knowledge, skill, and experience was not the concern for those Specialties, it was the "how to teach" that was important for me.
There are a lot of the Specialties you (or I for that matter) can be certified to teach based on the "20 dive" rule. The question to ask is whether you can provide value to a student even though you might be certified to teach based on the "20 dive" rule. Take photography for example. I think I am at the point where I could provide a good, solid Digital Photography course. However, we have another instructor who taught me the course. There is no way I could come close to providing the same level of instruction and value that he provides to his students. Therefore why would I bother becoming a Specialty Instructor for that course. I would certainly consider it if he wasn't with us.
Bill