This plugin adds a button "Sign in" to the user's hovercard and profile header to log in as member.
Full administrators have the permission per default. To set up users who can use the button, in your AdminCP go to Members -> Staff -> Administrators. Select the existing group or click on Add Administrator and grant access for System -> Members -> Can sign in as members.
For security reason, the button is not available on hovercards and profiles of the users that have access to AdminCP. For those users you can still use the default "Sign in as" feature in AdminCP.