Ask HN: What part of your work do you present?

We work in our codebases daily, so we have a very connected and detailed view of how our systems work. I would like to present some of my work, but I have a hard time thinking about what would make a good presentation. Some topics seem too small or obvious to be worth half an hour, and others seem impossible to explain in only half an hour. Rarely anything inbetween.

When you choose to give a presentation, how do you decide which part of the system to present? Do you find it hard to decouple the presentation from the context of why you had to build the system?