Sinking the lower wings of the ff holes on a Cello top. The ff's will be re-marked onto the sunken edge.
These wings, once the ff are cut, are a very important part of the top and care is needed not to weaken them when sinking the edges -
or to leave them too thick. They are a very delicate part of thefinished varnished top - in fact, the most delicate part of a violin full stop I think.
Many old instruments show damage here.