"not responding" means what, exactly?

As we all know, Application Monitor (as well as the Force-Quit dialog, and I think other places) shows those nifty "(not responding)" suffixes highlghted in red when a process is working ovetime or simply hung. I'm curious what triggers that, and how it can be captured from unix.

Here's what I think I know (from a lot of time spent on google and looking at man pages):

• the 'not responding' cue means that the app is not reponding to system calls for roughly 2 seconds (with no explanation of what system calls are being tried or how they fail)
• that Application Monitor mirrors the unix top utility (though there's nothing in top that correlates to 'not responding' - 'stuck' has a different meaning)
• that nothing in the ps utility corresponds to 'not responding'
• that this is one of those things that's going to drive me batty until I figure it out.

So what does 'not responding' in activity monitor *actually* mean, in unix terms? I'm not looking for a way to kill processes (I know how to do that), and the obvious things like top's 'stuck' indicator are wrong, and not what I'm after. I just want to know how I can leverage/mimic/replicate that 'not responding' thing that seems so simple in Application Monitor. Where-oh-where in unix (or anywhere else) can this result be produced?

DankeDankeDanke... 😀