Being able to dig in seemed to help me throw a knuckler. I never used it in a game because I could only get it to work about 10% of the time with a game density ball. The reason why I say digging in must help is that I can throw a very good one with a RIF ball. In Pustulio's thread he states that a harder ball is easier to throw. I was wondering if the majority of knuckle ballers would agree, just due to my limited experience.
I believe that if I were serious about getting a knuckler or had a student who wanted to learn it, I would use a series of different RIF balls. Go from the softest to the hardest then eventually to a real ball. As the student became able to throw with the softest ball with high consistency, I would mix in the next harder ball until he was throwing well with that one, etc. Eventually, with a lot of work, he'd be throwing it with a game ball.
The manicure definitely has to be right and finger position on the seams definitely seems to help with success level. I seem to get the best results from the crook of the horseshoe. Not the bottom and not the sides, but that curve leading into the base of the horseshoe. For whatever reason, it works best for me.