Scene: A typical saloon in the Old West.
An old north wind blows open the swinging saloon doors; tumbleweeds roll in from the street. A dark shadow appears in the doorway—it’s Killer Miller, come to settle the score!
Admittedly, my career as a screenwriter looks inauspicious. Setting aside the proposition that some might regard the sound of a trumpet as akin to the arrival of Killer Miller (not you, dear reader, nor I, but still it must be said), he is irrelevant to the analogy at hand. Instead, I am focused on those swinging saloon doors.
“Hinge” Mechanism. The doors—gates, really—are suspended on either side by hinges that allow them to swing freely, with a bit of spring that returns them to the center position. Let’s imagine a mechanism that adjusts the resistance: It can be set so light that a breeze will blow them apart or so strong that it would take a gale force wind to budge them.
The doors represent the vibrating points in the center of your embouchure, and the adjustable hinges represent the muscles (the orbicularis oris and others) in the corners of your mouth.
The job of the corner muscles is to position and hold the lips in the path of the airstream and apply just enough compression, so that passing air causes them to vibrate at the requisite speed and amplitude, equating to pitch and volume.
Delicate Balance. If insufficient compression is applied, the lips simply give way rather than vibrate, like hinges without enough spring to return the saloon doors to the center position.
If too much compression is applied, the lips stubbornly resist vibrating, like saloon doors that are wedged against each other.
The hinges must be strong enough to hold the doors in position yet allow them to swing freely. Similarly your embouchure must be firm enough to hold your lips in position, but not so firm as to inhibit vibration.
Too much compression is more common than too little, particularly as you approach the limits of your range. It’s a delicate balance that holds the key.