The Valsalva Maneuver is a body reflex that closes the airway in your throat with the glottis. This reflex is triggered when your body anticipates a difficult task that will require a lot of effort. When exercising, it is good practice to breath in, hold, and exert. But when playing the trumpet, you need to relax your throat and keep the glottis open to provide a good, steady air flow. This is necessary to get a nice, quality, fat tone. Typically, the Valsalva Maneuver kicks in automatically as you exert more effort to play higher notes. And this, in turn, means that you are working much harder with diminishing results. Your glottis is shutting off the air flow when you need it most!
A good way to become aware of keeping your glottis open is to exhale the air from your lungs, then push a little more air out. With the extra push, your glottis should be wide open. Notice how your throat feels? Do this several more times, then try breathing in more air and exhale with that same open throat feeling. This takes practice to overcome your body’s natural tendency to shut the glottis as you exert more effort to play higher pitches.
When playing the trumpet, just relax and blow while keeping your throat open. After getting a consistent good, open airway, try taking one note at a time up the scale. Listen to the sound you are producing. It should sound nice and controlled. At this point, don’t worry about the volume being too loud. Just play relaxed and open. Volume control will come with diaphragm pressure after you learn how to play with an open glottis.