In his response to my Cult of Sweat post, Rory said
"Boundaries have to be pushed judiciously, but this endeavor is all about pushing boundaries."
It's the "judiciously" part that I think is important. Boundaries have to be pushed, but you have to know how far to push them.
If you are a teacher, this clearly applies to how you treat your students. You must push them, but push them within limits. Anyone can be broken if you push hard enough, but breaking someone is not a sign of skill as a teacher. I keep coming back to Pavel's "do 10,000 jumping jacks" example: it is a "workout" that will break most people. It is also useless, unless you're trying to master the jumping jack for some reason.
But as a teacher, you also have to both be aware of, and push, your own boundaries. Not in terms of your own practice, but in terms of your teaching. If you never teach outside of your comfort zone, you will never teach. Period.
Teaching will force you to do push beyond what you think you can do or know. It will force you to answer questions you may not have thought of. And that's good.
But--you also have to know when pushing further is going to be too far. If you've never taught before, starting out with a 6-hour, 100 person seminar, is probably going to be disastrous. If you don't have the slightest background in groundfighting, you probably shouldn't do a seminar on groundwork. If you have never done any full contact sparring, you probably shouldn't add it the curriculum one day to "see what happens".
It's good to push a little further, but push to far, too fast, and you jeopardize your students. There's a balancing act in there (as there seems to be in most of this).
Of course, it occurs to me that most martial arts instructors are never taught how to do this, in any concrete way. Something to think about.