Definition and background:
A directive used with the accordion and other free aerophones that use bellows, to designate the rapid in and out movement of the bellows. This effect is similar to the tremolo on other instruments. The letters B.N. or the term " Bellows Normal " are the directives used to end this effect. It can also be notated by the double headed arrows on top of the pitch or pitches desired. Also B.N. , Bellows Normal.
For more information on the music term "Bellow shake" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary