What does Force Quit do in OS X?
I just probed this with dtrace. The PID of the program in question is sent a SIGTERM (i.e., signal 15) from loginwindow.

I also tried this with a stopped process (sent SIGSTOP), so that the process showed as "Not Responding" in the Force Quit dialog and got the same result, loginwindow sends a SIGTERM.

If you want to try yourself, run the following in Terminal:

sudo /usr/sbin/dtrace -n 'proc:::signal-send /pid/ { printf("%s -%d %d",execname,args[2],args[1]-&gt;pr_pid); }'


You'll then see the name of the process sending the signal, the signal number and the PID the signal is sent to.