This extension requires Core Design Scriptegrator plugin installed.

1. Install the plugin.
2. Open plugin configuration and don't forget to publish it.
3. Visit Joomla! User Manager and you can see the Login As icon on toolbar panel:
   

4. And now the funny part. Select user from your user list and then click on "Login As" icon. You get a prompt window with information about successfully "log-in". Confirm the window and you will be redirected to Joomla! frontend.
   Of course the new window opening is restricted by your pop-up block browser option.