Not sure how merge and push work

[Chris Seaton]

I was reading about the push command, and the help says

If you want to ensure you have the different changes in the other  
branch,
do a merge (see bzr help merge) from the other branch, and commit that
before doing a 'push --overwrite'.

What happens if there are local changes and remote changes, so I  
merge and then push --overwrite, but someone between the two commands  
pushed - what stops their changes from being lost?

Thanks

Chris