Here is what the salvation army tutor says about bellows, relating to the english concertina.
The bellows must be kept fairly level,and should be moved backward and forward by the action of the right arm, and should be drawn out and pressed in steadily, while the fingers manipulate the keys,but not pressed and drawn for each successive note.
The various degrees of force are all regulated by the action of the bellows.
    changing the bellows at slurs, chords double notes will give variety of effect.Care should be taken that in drawing and pressing the bellows they are not moved too abruptly or forcefully.
   most of that, I would say is good advice and I hope is useful.
      Dick Miles