Teaching is a skill, just like anything else in this world. You can be a natural at it or have no innate talent at it, but if you want to get better, you will. The teachers everyone remembers are experts both at their subject and at teaching. It's very, very hard to be an excellent teacher.

I don't know Jim and Joe's background, but (knowingly or not) they've hit on one of the most tried-and-true teaching skills for working with a group of students with mixed skill levels: peer instruction. The advanced students are forced to really push the boundries of their knowledge to explain the material, while the weaker students get a second (and lower pressure) presenation of the material, often with a different approach. If you know nothing else about teaching, this is probably one of the best techniques to keep in your back pocket.