All in or nada

[Laughingboy]

I've seen a lot of posts about switching to all-in or nothing mode in later stages of the tournament, but I'm a little hazy on the exact criteria. In particular, let's say BB is at 200. You're in second on the button with 4k chips. SB is the leader with 4900k chips. BB has 800 left after posting. Say you have a hand like A-9S. Do you Raise just enough to put the bb all in? Push all in yourself? Something else?

By the way, don't get caught up with my specific example. What I really want to know is when you might want to just put the short stack all in, but might consider folding a push from the big stack.

Thanks!
Sean