Documentation : how to "undo" hg up

[Patrick Waugh]

Paul,

On Wed, Sep 17, 2008 at 3:58 PM, Paul R <paul.r.ml at gmail.com> wrote:
> Hello Patrick,
>
> On Wed, 17 Sep 2008 15:15:49 -0500, "Patrick Waugh" <ptwaugh at gmail.com> said:
>> What does "flush the working copy of a repos" mean? Patrick
>
> I meant what ``hg update null'' does.
> "Un-ckeckout", maybe.

That's like telling me "this means this"!  I could figure out that
much myself.  =)

Can someone explain what this command does, perhaps with an example?

Patrick