I was looking for git HEAD concept and I came across the Atlassian git tutorial (checking out and reverting)
Now let us execute git checkout b

This is an update to the "Commit History" tree
What does it mean 'This is an update to the Commit History'? This doesn't touch at all the Commit History, just takes out a specific commit into the working tree and, the most important thing, lets the subsequent commits to attach to the HEAD (and this seems to create an unnamed branch that should be avoided in most cases. The most common use case for this situation, called detached HEAD is for reading purposes).