explain uncommit to me please

[Chris Hecker]

I just used bzr uncommit for the first time tonite, and I feel a little 
dirty.  :)

I wanted to change the log message I just committed, and saw uncommit, 
and said what the hell.  What happens to that revision in the branch? 
It's still got a revid, so it's in there somewhere?  It doesn't show up 
in log or anything.  If I try to log -r revid:blah I hit the crash bug 
referenced here 
(https://bugs.launchpad.net/ubuntu/+source/bzr/+bug/409266).  I assume 
it's still in there somewhere?  What would have happened if I'd pushed 
to another branch before uncommitting?  Etc.

I have a feeling this has something to do with multiple heads or 
something like that, that I don't really understand since I'm pretty new 
to dvcs.

Thanks,
Chris