the undo system off, all the recorded actions are discarded, and the undo buffers are reset.
If you call this command with no arguments, it will toggle the flag. If you
specify 0 or 1, the flag will be set to false or true, respectively. A lower
u' will appear on the status bar if the flag is true.
The usefulness of this option relies in the fact the undo system is a major memory eater. If you plan to do massive editing (say, cutting and pasting megabytes of text) it is a good idea to disable the undo system, both for improving (doubling) performance and for using less (half) memory. Except for this, on a virtual memory system I see no reason to not keep the undo flag always true, and this is indeed the default.