How is the undo tree used in Vim?

This page explains everything you need to know:

http://vimdoc.sourceforge.net/htmldoc/usr_32.html

I'm a bit late to the party,
but I figured I'd mention that I wrote an undo tree visualization plugin for Vim :
https://github.com/sjl/gundo.vim

Personally I found that graphing the tree like this was the only way I could make sense of it.

See also :h undo-redo, which lists all the commands and their usage.

There are two ways to traverse the undo tree. One is to go "back in time". g+ and g- will traverse all of the nodes in the tree in chronological or reverse-chronological order (which can be a bit confusing, because it can jump arbitrarily between undo branches, but if you do g- long enough you'll always get where you need to go eventually). :earlier and :later take a time descriptor like 7m or 1h; again this can jump you arbitrarily between undo branches.

The other way is to jump to specific nodes in the tree using :undo n where n is a number of an action. (All actions, i.e. text additions, deletions, replacements, are numbered sequentially as you do them.) You can look up the number of the actions on the leaves of the undo tree via :undolist. This will let you jump between branches easily. You can then use u and Ctrl-R to move up and down that branch.

There are some good examples in the Vim help. The best way to figure out how this works is to play with it a bit.

If you're using vim, you can navigate through the undo tree using:

• u: (undo) move back in the undo tree
• Ctrl+R: (redo) move forward in the undo tree

Other ways of bringing document back or forward in time:

• :earlier 15m: move back in time 15 minutes
• :later 15m: move front in time 15 minutes