I think, rather than fast and slow (tempo), you mean more or less 'energy'. In most styles, changes in energy occur because of the effect of the part markers (A = blue = normal, B = green = busier) for substyles. The bass may 'walk' / play doubletime in B, for example. Also, the default is to perform the first time through with blue feel, the middle chorus with green (though you don't see that) and return to blue for the last time through. You can add these part markers wherever you like by left clicking on a measure number. They will also cause a double bar in a printout. Try it.

If you don't want BIAB to change the energy in the middle choruses, go to Opt., Preferences, Overrides. Uncheck the box that says, "Vary styles in middle choruses". Then you could do Edit, Unfold to make your whole song visible, where you can add the part markers to produce exactly what you want.

Note: some MIDI styles go well beyond just two part markers, supporting variations for C,D and beyond. If you see other colors than blue and green, you have one of those styles.



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