Glissando/Pitch Bend is the smooth transition from one note to another by bending the pitch through an interval with smooth finger movements or by covering and uncovering the embouchure hole with lip movement. Most often a combination of these is used. This technique can be applied to normal flute tone or any variation of Aeolian Sound. Composers should note that a flute is not capable of making the same type of incredibly smooth, multi-octave Glissando as a string instrument.
The interval size of a Glissando done with only lip movement varies depending on the direction of the Glissando, the pitch and octave of the note, and the player's lip flexibility. On the short air column (left hand notes) there is more flexibility than in the long air column (both hands). A Glissando using primarily finger movement can cover a much larger interval size and varies depending on direction and notes in the Glissando, and the player.
Glissandi/Pitch Bends are produced by using sensitive, smooth finger movements, or by covering and uncovering the embouchure hole with lip movement.
Lip Glissano/Pitch Bend: With practice a Lip Glissando/Pitch Bend can lower a note by a minor third and raise a note by a major second, depending on the specific pitch and octave. The primary way this is done is by covering and uncovering the embouchure hole with the lips, changing the shape of the mouth cavity to resonate lower or higher pitched vowels, changing the size of the embouchure itself, and lastly, by rolling the flute in or out a bit. You can also experiment with lifting and lowering the soft palate. Any given Glissando/Pitch Bend will require a combination of these techniques for bending the note. Try to avoid only rolling the flute in or out as this severely limits the control and size of the Glissando interval that you can achieve.
Fingered Glissando/Pitch Bend: There are two ways of achieving a larger FIngered Glissando. The first of these is to slide fingers off of open-holes and slowly release the key(s). The second method is to use alternate or "fake" fingerings. This is especially helpful in the upper register notes. For larger intervals, you can start to Glissando on one note with a combination of fingers and lip movement, diminuendo al niente, and approach the end note with a Glissando (fingers and lips).
Lip Glissando: Down and Up