I was always impressed by guys like Albert Zamora who do those quick bellows triplets. My cheap Chinese accordion was way too slow and I couldn't do anything along those lines.
Now that I have a Hohner, I am starting to get the idea.
A few pointers for anyone interested.
1. You can pretty well do this technique with any chord, seventh, sixth, note etc.
2. This is a quick out - in - out action. You need to use the air release button. The trick is to push the bellows completely closed on the in of the out - in - out. Use the air release on the in, letting a little air escape while you are sounding the note, chord, whatever.
3. A good way to control the bellows is to not open completely. It is kind of tricky but what you essentially want to do is keep the front of the bellows closed and only let the back open, as if making the whole accordion into a "C" shape. This way it is easier to get the accordion closed again after each triplet.
4. The right hand comes into play here also. You can't just sound the chord and do all the work with the bellows and the air release. After each triplet, release pressure on the buttons as soon as you have sounded the third beat of the triplet. You could call it muting or cutting off the chord. This takes tons of practice to develop into something really fast.
Listen to Albert's great hit "Preita Casada" to hear some great bellows technique.
I just started to practice this technique so this is not expert advice, I'm mostly relaying what my teacher told me. Of course he can play this technique like crazy, so I expect this is how it is done.
Another thing that is cool is that if you run out of air by extending the bellow completely, you can use this technique to continue playing while you close the bellows more and more each triplet.
Good luck with this one.