Hi Lionel,
It's a dead simple hack that I wrote it in a manner of hours. A friend and I have tested it on 4 different boards for a couple of hours as well, on which it's officially live as of today to the members' joy.
If you're worried about security, there's nothing in the hack that makes the board more insecure/secure than it was before. The user's current session is checked against on switching.
If you're logged on as a master account you can switch to your slave accounts, and if you're currently logged on to a slave account you can switch to your master account (or possible additional slave account).
Simply trying someone else's userID in the URL parameter will fail to achieve anything
Feel free to take it apart on a test installation if you care to examine this for yourself. As a matter of speaking this is a good thing to do for any hack whatsoever by anyone. Tinkering with productions boards should be approached with caution, as always.