When you use the Force Quit dialog from the Apple menu or Command+Option+Esc, you are actually sending a signal, called SIGKILL, to the application you chose in the list. This signal forces an application or process to end, regardless of what it's doing.
There are many more signals besides SIGKILL, such as SIGTERM (quit), and SIGABRT (abort). Many of these are useful when programs freeze, yet they are normally difficult to use and require typing commands in the Terminal to reach.
SendSignal is a simple application that allows you to send signals to any process, and you can run it as root if you wish by choosing "Yes" in the box when you launch it. It's useful for freezes, debugging, and other uses.
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