1. Make sure you've checked out the branch you're replacing.

2. Assuming that master is the local branch you're replacing, and that "origin/master" is the remote branch you want to reset to:

This updates your local HEAD branch to be the same revision as origin/master, and --hard will sync this change into the index and workspace as well.

Basically, @{u} is just shorthand for the upstream branch that your current branch is tracking. For example, this typically equates to origin/[my-current-branch-name]. It's nice because it's branch agnostic.

Make sure to git fetch first to get the latest copy of the remote branch.

Side Note: In most shells the command will work as written, but if you're using Git in PowerShell or csh you'll need to escape the special characters first, though I've also confirmed it works as a string in both, so for example: see comment ↑ (#git reset --hard '@{u}')