The retrospective is to me the heartbeat of Scrum. If you get that right everything else will work out fine over time. Because the retro is based on the idea of Kaizen as part of the Toyota Production System and the principle of continuous improvement, I believe it is helpful to look at some of the other ideas laid out in The Toyota Way, and how those ideas can (and should) be brought to bear in Scrum, and how Kaizen embodies all of them under one umbrella term.
Hansei is an aspect of Kaizen which is critical to success. Hansei means “self-reflection,” and in Japanese culture it means to acknowledge your own mistake and pledge improvement. Without the idea of Hansei, a retrospective can become focused on all the things outside of the control of the team and team members to address. There are substantial gains to be made by the practice of Hansei. Most spiritual and religious practices have something similar to this form of self-examination, and it is a wise practice even in the secular world to examine oneself critically and determine where things could be improved.
This is one reason why the Scrum retrospective should be the one absolutely protected ceremony in Scrum. No one but the team should be allowed to participate because it should be a very safe space for Hansei. Especially no managers (who should not even be on the team in the first place).
The retrospective would actually be called Hansei-kai or “reflection meeting,” and it happens every sprint at the end. Even when things have gone very well, maybe even seemingly perfect, there should always be problems to address, or as a Toyota manager would put it, “No problem is a problem!” Because if you objectively and critically evaluate the work, you will either find problems to be dealt with, or it means that you did not stretch to meet or exceed your expected ability.
One of the fourteen main principles of the Toyota Way is to “become a learning organization through relentless reflection (Hansei) and continuous improvement (Kaizen),” and from this perspective, the Scrum retrospective should help drive organizational learning at a team level, and hopefully, the team will also share their findings with other teams to increase their learnings as well.
It is very educational to understand the roots of where the Agile Manifesto and Scrum come from, and the study of the Toyota Way is crucial to that understanding. In fact, there are many more lessons there than are contained in most Agile frameworks and practices today. In other words, I believe we still have a long way to go to catch up with what made Toyota the number one car manufacturer in the history of the world.
Happy learning!