What is a lip trill, and why do singers do it?

A lip trill is singing a note or a slide while your lips flutter loosely, like a horse blowing out or a child imitating a motorbike. It only works when the airflow is steady and not too strong, which is exactly what makes it useful: it balances breath and voice, keeps the effort low and lets you slide through your range without squeezing. If your lips will not keep fluttering, press lightly on your cheeks with your fingertips to take some of the tension off.

A guided warm-up with lip trills, timed, with a sung example