It's so nice teaching a course for the second/third/fourth time because you really get to know what the keystone understandings are that will help students learn something. For the area and volume unit, it's the idea of the accumulation. This example (above) in particular I think explains the idea really well. We had a nice 30 second discussion of why it makes sense that this integral simplifies to just the integral of 3 from 0 to 2pi. I can just see how if I taught Calculus another 10 times I would be able to find these crucial examples for nearly every topic.