Method
Although listed here as a contemporary technique, lip bends are actually well-ingrained within the brass players’ standard practice. This stems from the fact that the conscientious brass player continuously performs micro-adjustments to their intonation, often taking the forms of slight lip bends one way or another. In this technique, the lips and muscles of the embouchure are either slackened or tightened subtly to lower or raise the pitch. Depending on the range, these lip bends can vary from as little as a microtone to as wide as a perfect fourth (or beyond).
Necessary information
Lip bends are most noticeable in the lower five partials of the tuba and euphonium, as the wide spaces in these partials allow for lots of embouchure wiggle room. They are certainly achievable in the higher range, but the reduced space between those partials only allow for so much room before a completely different partial or split tone is activated.
When performing large lip bends, the subjective tone of the note tends to become unfocused, due to the distortion of the normal embouchure placement.
Notation
Lip bends are often notated with the use of small slur lines in the score (for indeterminate lip bends), or with an explicit “lip-glissando” for explicit lip bends. See the example below for both styles.
Relative Difficulty
Intermediate
Works to consider (bolded titles are particularly representative examples of this technique)
Three Essays – William Penn
The Composer's Guide to the Tuba 