This might be a little out of context but when I first started out, and by all means I'm still a novice at this art, I got some cheap wood and deliberately made the bows in a fashion that would cause exaggerated hinges and other things that would constitute bad tillering. I found at the beginning I was not seeing certain aspects such as were the wood was bending and not bending, I wasn't seeing obvious hinges, and twists. I do now. I just used some cheap pine that was laying around and made a whole bunch of firewood bows. It has helped a lot with my tillering. My reasoning is that we are very use to seeing bows with great tiller here and on other sites, but we very rarely see examples of poor tiller in bows. By deliberately making mistakes I found how what these issues look like, how they occur and what not to do if I want to tiller a bow correctly.