Check out this cool video on straw phonation from Dr. Ingo Titze.
A respected vocal scientist, Dr. Titze demonstrates vocal exercises that consist of creating sounds into a straw. After a few minutes of these exercises, your voice will feel like it is coming out of your eyes (a buzzy feeling in your face) which he explains is the optimal placement for your voice.
He sings the National Anthem into a straw, which, he explains, helps to strengthen and train the vocal folds for the best transfer of power from the glottis to the lips. He demonstrates the "semi-occluded" phonation which balances the air pressure around the vocal folds: sub-glottal (below) and super-glottal (above). Examples of these semi-occluded phonations are the raspberries and lip buzzing sounds you recognize from our tips and your vocal coach.
Cool, huh? This will all make more sense when you watch the video!
Here is an article Dr. Titze wrote explaining the concept in more depth:
"OVER THE PAST TEN YEARS John Nix and I have been describing vocal exercises that utilize phonation into a straw.1 These exercises are part of a bigger picture of training vocal fold adduction, registration, and epilarynx tube narrowing for the best acoustic power transfer from the glottis to the lips. Use of a thin straw is not the only way to facilitate this power transfer, but a semi-occlusion at the mouth is a requirement. Lip trills, nasals, / u / and / o / vowels, bilabial fricatives, and other semi-occlusions can all be used to meet the objective. 2"
By Titze, Ingo: Academic journal article from Journal of Singing, Vol. 66, No. 5