He supposedly gets it when Mace collapses part of hist chest.
However I may point out that this is NOT CANNON.
It is also incongruent with EIII as Greivous' chest is clearly intact and if you could just collapse him like a aluminum can, jedi would have done that already.
Basically, Lucas just has a fetish for asthmatic villians.
I always thought the Clone Wars were considered Cannon
It would take a Jedi who walks the line between Light and Dark yet not go off into the Darkside to use a Darkside Force-technique to do that. Mace fits all of those catagories.
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"Today was good. Today was fun. Tomorrow is another one." -Dr. Seuss
Since Lucas endorsed it and it was being shown on the official website, I also considered The Clone Wars series Canon(remember, they also showed how Anakin got that scar). I'm fine with the idea that the second best Jedi in the galaxy would be able to do that to Grievous. As far as why it wasn't crushed in the movie, he could have gotten a replacement chestpiece.... you can't replace crushed organs or damaged lungs though.
__________________ "I feel lethal, on the verge of frenzy."
I would assume so becuause if someone got some random good hit in, he would probably need medical attention quick, not have to wait to enter another star system and get it there. Although, I don't think he got a replacement, he was pretty busy throughout the end of Clone Wars series and the beginning of Episode 3 it seemed.