Right now I'm looking at the tab for Santeria by Sublime, and I'm wondering what exactly upstroke ska is. I mean, I know it's a way of playing the chords, but could somebody give me a detailed way of playing the chords? They don't sound choppy enough when I'm playing them.

I wasn't sure where to post this, so I guess this is a good place.
muting on the downstroke and then letting the upstroke ring out. its a rhythmic thing you gotta get used to.
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you basically do a muted downstroke on the downbeat, and then do an un-muted upstroke on the upbeat.

EDIT: and most of the time it's not palm-muting.
I always thought:

You do them using barre chords; like this:

1 & 2 & 3 & 4 & 1 & 2 & 3 & 4 &
U x U x U x U x U x U x U x U x

U = upstroke
x = mute strings (Lift them up a touch) and down-strum

-- I might be wrong, but I think some people just play the highest 3 strings of the chord to get that "choppy" sound.
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I don't play ska or anything, but I've fiddled around with the rhythm and this is what I do:

1) I don't play with a pick.
2) I only hit the top three strings on the upstrokes.
3) On the downstrokes, I just hit the strings with the open palm of my hand.

It works fine for me.
It's just hitting the top three (thinnest) strings on the upbeats. Make sure you stop the notes by a mixture of palming the strings and lifting pressure from the strings with your fret hand. That will make sure there is no sustain on the notes and just continue. It take a bit of practice.