I thought it would be neat to post some info showing how the accordion played that part on "Wouldn't It Be Nice" which has been identified as the "triple bellows shake". Disclaimer: I do not own an accordion, after borrowing a student model from a pro who recorded a song with me some time ago I'm convinced I couldn't do it justice without intense practice, but maybe if it hasn't already been done, an accordion player will post a similar tutorial on how to play that exact WIBN part. Until then, a demo of the technique itself has to suffice.

Frank Marocco:
"He (Brian) would have me try different things. He would say, 'That's what I want,' 'Do this with this sound!' On 'Wouldn't It Be Nice' I ended up doing something that was the most difficult thing I've ever done before or since. I came up with this part that was like a triple bellow shake. It was physically demanding, because we had to do it over and over again, and by the end of the session, it was exhausting. I remember thinking, 'I'll never suggest that again!'I think there may have been some misunderstanding on how it came about in the studio, maybe from the wording used in various interviews and clips. The "bellows shake" is an established technique for playing accordion, and the "triple bellows shake" is a more complex and difficult variation of that technique. Sometimes it felt like the descriptions of that session suggested the technique itself was actually developed and maybe first used at that session, again due to some of the wording in interviews and whatnot, when really it is just a very complex variation of the accordion player's "bag of tricks", of course a more advanced technique and a physically demanding one.
You could compare it to any other instrument, like the guitar: If a song called for something like a mandolin-like sound, the guitarist would most likely reach into his/her bag of tricks and use tremolo picking combined with certain notes high up on the fretboard to suggest mandolin. The same thing seems to have happened on WIBN, where a certain sound was asked for, and the musician called on a technique he had mastered to fill that request. It wasn't necessarily a case of developing a new technique out of the blue and on demand, but expanding on an old but difficult one.
So you *could* say Frank Marocco made an already difficult technique even more challenging, or put a "new" spin on it...

The impression I got - again, not being an accordion player but having had one on loan to mess around with some time ago - is that the specific "triple bellows shake" technique produces triplet figures, rhythmically. It would fit WIBN since it is a shuffle based on triplets, but I'm curious if after watching this lesson video that is what we hear on the actual track, or whether the modification-variation Frank hinted at was changing the rhythms produced by the triple bellows shake.
The exact demonstration of the triple bellows shake is found at 1:47 on this lesson video:
http://www.youtube.com/watch?v=wUt6ObU_UFYShort of having a pro actually showing us the exact WIBN part, I thought it was cool to see someone break down the technique itself and we get a different view of what exactly went into making that incredible record. And it's good to also put a visual to the sound we hear, to also appreciate what the musicians were actually doing for those hours in the studio while the record was being made, and in the case of the accordion how much technique and skill was required, not to mention the physical aspects of being able to play something like this consistently take after take.
Remember too that Brian had two of the finest accordion players in the world playing that part, Frank Marocco and Carl Fortina!