I'd agree most whistling is "low pressure," but the "loud (fingerless) whistle" uses a lot of breath support and the use of many of the muscles, etc., that the trumpet embouchure uses. It can be as screamin' and almost as loud as a trumpet and requires a lot of breath and effort to be optimum.
Originally Posted by rowuk
Here's one explanation at http://www.natwilson.com/stuff/whistle.html that sounds a bit like someone giving instructions on the horn except for the lips being apart to let out the air. I might have described the process differently, but it demonstrates the point:
The fingerless whistle is a natural outgrowth of the fingered whistle. In the first method, you use your fingers to keep the lip taut and in place. With the next method, you remove your fingers and don't use them at all (except to cross them for good luck). Instead of using your fingers, you rely on your muscles in your lips, cheeks, and jaw. Since this technique requires greater control of those muscles, it may be easier to master the fingered whistle first, and then move on to the fingerless method.
Draw back lips
Begin by extending the lower jaw slightly, and pulling the corners of your mouth back a bit, towards your ears. Your bottom teeth should not be visible, but it's fine if your upper teeth are.
Your bottom lip should be quite taut against the lower teeth; if you have need help with this movement, press an index and middle fingertip on either side of the mouth to draw the lip slightly out to the corners. Note: this action is not an insertion of the fingers into the mouth, as the first method indicated. In this instance, you're simply stretching the lower lip a bit, and the fingertips aren't in the airstream.
Draw back the tongue
Now comes the crucial part of the whistle.
The tongue must be drawn back so that it sort of floats in the mouth at the level of the lower front teeth. This action also broadens and flattens the front edge of the tongue, yet there's still a space between the tongue and the lower front teeth.
The sound of the whistle comes from air that is blown over a bevel, or a sharply angled edge. In this case, the sound is created by the upper teeth and tongue forcing air on to the lower lip and teeth.
Steps 2 and 3 follow each other very closely, if not simultaneously.
Inhale deeply and exhale--the air should flow under your tongue, up through the space between the tongue and teeth, and out of the mouth. Experiment with the position of the fingers, the draw of the tongue, the angle of the jaw, and the strength of your exhalation.
Start off with a fairly gentle blow. You'll produce a whistle of lower volume, but you'll also have more breath to practice with if you don't spend it all in the first three seconds.
Using your upper lip and teeth, direct the air downwards and towards your lower teeth. The focus of the air is crucial for this technique--you should be able to feel the air on the underside of your tongue. And if your hold your finger below your lower lip, you should feel the downward thrust of air when you exhale.
As you blow, adjust your tongue and jaws to find the sweet spot. This is the area of maximum efficiency, where the air is blown directly over the sharpest part of the bevel. This results in a strong, clear tone that's constant, as opposed to a breathy, lower-volume sound that fades in and out.
Listen for the following: the sound you'll start with will sound as if you're letting air out of a tire. Every now and then, the clear and full tone will come through, and you'll know that it's only a matter of time before you're hailing every pet and taxi in your community.