Interstitial Journaling calls for journaling about what you just finished working on. Often, you’re still thinking about that project. Write those thoughts down, empty your brain.
It's interesting, in the past I would have considered journaling about the project I just completed as useful notes to have for the future when I picked the project up again; but here the author focuses on the benefit of having your mind clear for your next project. Both are valuable, but I like the immediate benefit in the present, as it motivates to do it right now, instead of for some unsure reward in the future depending on if the project is ever picked up again.