Easily the most misunderstood aspect of brass playing is what is "supposed to be happening" with the lips and embouchure in general when we play. For years we have been told that our lips are supposed to be buzzing at all times when we play any notes. In fact, the lips must vibrate but NOT in the close configurations as when we do lip buzzing. The air, once compressed, must have a pathway to be released in a controlled manner from the body. The air actually initially aims at the surface of the top lip, hopefully as far forward as sensibly possible. When it moves at a high velocity, it causes the lip to vibrate from the impact of the air hitting it. The air doesn't just go past the lip, it "spins" into an eddy (a kind of whirlpool) which "bounces downward" as it spins out of the eddy and this downward movement activates a vibration on the surface of-the-bottom lip which also then spins into another eddy. These are called vortices (vortex is singular) and they are the basis of a "sympathetic vibration" occuring between the two lips...which IS WHAT PRODUCES SOUN D. The closer you put your lips together, the softer, smaller, thinner, or more "pinched" your sound will be. When you open up the size of the aperture, the first thing you will notice is the freedom of the movement of the air, then the opening up of the sound. Once the aperture is opened, the player must also increase the tension in the ab support to increase the air flow which in turn must fill the larger gap in the aperture opening. This forces the player to USE THEIR AIR which IS the more efficient way to play. All people ever taIk about is AIR but then the confusion hits when they try to explain how it works and what the player is "supposed to be doing". Soft, delicate playing requires that the player close the aperture down as the airflow is also dimished but understand enough to know that when you "shift gears or hats" as a player into a more demanding situation such as playing lead trumpet, the key is to balance the support and air flow with the aperture. These aperture muscles need to be developed properly as well. The best exercise I know for this is lip buzzing as long as the player doesn't start to confuse the tightly pursed lips necessary in lip buzzing with what is necessary and different when actually playing. Lip buzzing also must not be done in long hard sessions. It is best done conservatively, usually 30 second sessions done around 10 times a day, alot less to NONE on busy playing days.