It is just a byproduct of the fermentation process. It means things are going right inside that little tin, and it is visual validation for us fools who let tobacco's sit in our cellar's for years and years and years.
That is a very good way of looking at it! It is kin of like the little red pop-up timer on your turkey
I tried to take some pictures, but it is kind of hard to see tell.
Here you can kind of see the tab starting to pop up on its own
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