Activity diagrams. I find that they are my go to when I want to communicate how something will work. This is when we are considering a modification of something that already exists or if the idea is new but well defined.

I'll use draw.io usually but if it's to clarify something small, I'll just draw it on my notebook and snap a photo of it.

That's what I love about the activity diagram. Communicates a ton of information in a form that even if you've never seen it, it will be pretty intuitive. And creating it is also super lightweight.