They are basically like tiny reamers, but they have to operate from inside the flute.
They are passed up the bore and lined up underneath the hole to be undercut. I use a wooden dowel with a tiny magnet in the end to do this. Then a T shaped handle is screwed into the threaded bore in the fraise, and this allows the tool to be pulled up into the tone hole and rotated. The thread is cut so that the direction of cut also tightens the handle in the fraise. With sufficient material removed, the handle is removed by pulling up hard into the hole and snappily reversing the direction of rotation. The fraise is then tapped free and falls back into the bore.
Here's a few pics of the manufacture.
The process requires an engineering lathe, and a milling machine with a dividing head. The threaded bore and basic taper of the tool are produced on the lathe, and then the flutes are cut on the milling machine, using the dividing head to ensure even division of the flutes. The image below shows the flute cutting, in this case at intervals of 30 deg. giving 12 flutes. ( I should point out here, to avoid confusion that the grooves in a drill or reamer are called flutes )
Since the tone holes on flutes are a variety of different sizes, this necessitates a variety of fraises, in different sizes and tapers to produce the desired result.
Here's three that I made last weekend...
Two interesting facts about the fraise. They are fiddly little buggers to make and fraise is the french for strawberry...