The bass buttons triggers the bass mechanism.
Here's part of the bass mechanism. The white button opens a valve that bypasses air in order to adjust the air level in the bellows. The bass mechanism produces both bass and chord notes. This accordion has 96 Stradella bass buttons (6 rows *16 buttons in each row).
Two of the rows are used to produce bass notes. There's essentially only one octave (twelve notes) available when you're playing bass runs. Most accordions, however, have several sets of reeds that may be sounding together when a single bass button is pressed. This particular accordion has four sets of reeds for the bass notes. There are no register buttons so all four sets are always used simultaneously.
Four of the rows are used to produce notes to be combined into chords. The bass mechanism again essentially has only one octave (twelve notes) available. Usually three or four notes are used together when the four types of chords - Major, minor, 7th and diminished - are produced. Other chord types are possible by pressing more than one chord button. This accordion uses two of the four sets of reeds for each of the twelve notes used to produce chords. Since there are no bass register buttons, the two sets are always used together.
I didn't remove the bass mechanism to take pictures of what's behind since this looked rather complicated. The same type of levers and valves are used as on the treble side. I did however take pictures of the reed blocks on the bass side.
On this accordion there are two reed blocks on the bass side. Each reed block has two sets of reed plates for 12 notes each. When chord notes are produced, only one of the blocks is used (two sets of reeds). When bass notes are produced, both blocks are used (four sets of reeds). The reeds and reed plates are described in more detail in the treble side section.
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