I've actually not tested that feature yet (busy), but it appears if you type in a users name (other than your own) it should allow you to use the forums with the same permissions as that user you entered.
Obviously you need to have another user account besides admn to test that.
For those who object to having this feature I'm bewildered.
If a user has a problem I can investigate MUCH faster by logging in as them direct from the adminCp -
to logout as admin/ login as user/check what the problem is / logout/ login as admin maybe contact them to get more info OR try a fix/ sometimes logout as admin and login as user again to check - CLUNKY.
While we're on it, remove the option to change a users password as well. And encrypt private messages. Etc.
There will always be the possibilities to do bad stuff, that doesn't make the tool itself bad. I've had users who couldn't log in, had permission problems, etc. which I couldn't find all that easily in vBulletin because of all the permissions being spread across the admin CP. Using the login to account feature makes it way easier to troubleshoot, and see what's actually going on.
Anyway, XenForo's solution is totally fine too and I like that it's already in there by default.