This step is often overlooked. Everyone knows about research and action, but how many people learn from their mistakes?

That is the basis of synthesis. You take the precis that you wrote in your research stage, and take the results of your actions and mistakes from your action stage and re-write a little precis that summarizes what you've learnt.

Note that this time when you write the summary, the information you wrote from the initial understand phase should all be in your head, ie. you shouldn't have to look at what you wrote. That's another sign that something has been understood by the way; there is no need for memory, all the lessons have already been absorbed from your actions.