You tap the wrong point, or apply a protocol to the wrong visit, and the chart has already saved it. There is no Save button to avoid, so Undo is how you take the click back.
Where it lives
Undo, Redo and History sit together in the header of the chart editor. On a phone they are behind the three-dot More session actions menu on the bottom bar. They are buttons only: Goji adds no keyboard shortcut of its own.
Two jobs, cheapest first
Notes save about five seconds after you stop typing. While that timer is running, Undo cancels it and puts the note back to what was last saved, so a paragraph you did not mean to write never reaches the record. Anything you changed by clicking, such as points, pulse, tongue, pattern, technique or a custom field, was already saved the moment you made it, so it is not part of that window.
When nothing is waiting, Undo steps back one saved version. That includes applying a protocol or copying the last session, which each save as one step, so one click reverses the whole thing. Keep clicking and it walks further back, one version per click, until it says Nothing earlier to undo to. Undo stays disabled until the session has an earlier save to go back to.
Redo, and what it is not
Redo walks forward through the versions you just stepped back over. It is available only right after an Undo. Make a new edit, or restore from History, and it goes away, because the version it would return to is no longer the next step. Redo also disappears if you reload the page, so if you need a version from before an Undo after a reload, open History.
What it leaves in the record
Undo is a shortcut to the same restore that History offers on each row, so nothing is erased. Each Undo and Redo is saved forward as a Restored row naming who did it, and the versions you stepped back over stay in the list. Undo steps by history row, so it also counts the rows for a status change or signature. Those rows carry the same chart content as the one before, so a click can land on identical content. Restoring never changes whether the visit is a draft or complete; that stays as you set it.
On a signed chart Undo and Redo are disabled, and a restore is refused on the server as well. Unsign first. The steps are taken from the session's own history, and it lists every team member's saves, so Undo can step back past a colleague's change as well as your own.