Half-holing is a technique familiar to recorder players. In order to move some notes up or down by a semitone it is necessary to partially cover a hole, allowing some air to escape through it. In fact to play in the upper register at all it is necessary to jam the tip of the thumb into the hole in order to force the instrument to overblow by an octave.
This is not a problem faced by players of the orchestral flute which has a system of keywork that makes it fully chromatic. But the recorder is relatively sophisticated in comparison to the humble bamboo flute. The tone of a good bamboo flute is both stronger and richer than that of a recorder but playing one with other instruments can present a problem. A recorder is, at least in theory, fully chromatic. Not so the bamboo flute. My favourite is pitched in Ab - not a great key for spontaneous jams around the camp fire.
The other day I was playing a studio session for a maker of library music. We had used the orchestral flute almost exclusively but thought we try something more 'ethnic' for a change. I have one in C which is close enough to the key of F, the key of the piece in question. But it meant half-holing the top hole in order to make a Bb.
This was not a problem until I had to execute a fast descending run. To ensure accuracy of pitch I partially covered the hole with masking tape. As you can see in the picture, half-holing is a misnomer - nearly all the hole is covered.
If you a are a flute (or recorder) player reading this I should explain that although the flute in question is pitched in C it feels like I'm playing a D scale. This makes it a transposing instrument, a fact that became clear when we abandoned the bamboo and went back to orchestral flute. I then had the sensation of reading everything a tone down. Flutes and whistles in D are very popular because they make the notes any flute or recorder player would expect. D is also a key that most guitarists can manage without too much trouble.
It is possible to cover holes lower down the instrument to facilitate playing in other keys. I part covered the second hole as well (pictured) to give me C Dorian. Always cover the side of the hole nearest the wrist of the playing hand - this allows glissandi and other effects that make these instruments so wonderful.