Emacs' Undo

Here is how undo works in most of the editors. For each change, a state is recorded in some internal structure:

s1 --> s2 --> s3 --> s4 --> s5
                            |
                            last state

You can undo by going back in time, say to s2:

s1 --> s2 --> s3 --> s4 --> s5
        |
        undo reinstating this

You are still able to move forward in time (that is, redoing):

s1 --> s2 --> s3 --> s4 --> s5
               |
               redo reinstating this

But as soon as you make a modification, you break the undo history:

s1 --> s2 --> s3 ~~> s4 ~~> s5  // oh no! This branch is gone
                \
                 sx --> sy

Emacs won't delete this branch.
It undoes by appending new operations. In this sense, it is closer to git revert than git reset. So, if this is your undo history:

s1 --> s2 --> s3 --> s4 --> s5

undoing up to s2 means restoring s4, s3 and s2 re-appending them to the history:

s1 --> s2 --> s3 --> s4 --> s5
                              \
                               `--> s4 --> s3 --> s2

So, if you make changes from there, no information will be lost:

s1 --> s2 --> s3 --> s4 --> s5
                              \
                               `--> s4 --> s3 --> s2 --> sx --> sy 

Emacs ships with a little built-in package that displays this as a Git-like history tree:

s1 --> s2 --> s3 --> s4 --> s5
               \
                sx --> sy

and lets you navitate the history:

Jujutsu provides similar features with the subcommands of jj op.