What's a B/C box?

Ooh Rees, You Cat!
Seriously though, just to add my amateur opinion,
I find it helps if, as George advises, the bellows are kept as closed as is practicable but also that the accordion is kept still, i.e, by using two straps...... (Uh-Oh, Here we go again........ The Two Strap Brigade

), it just means all my concentration is on playing the correct notes, Not shared with holding the box still.
Skipy