Trumpet players know this story all too well.... To get those screaming high notes, there is a natural tendency to push the mouthpiece HARD against the chops. Often times too hard - to the point where you regret it later. Players know that more mouthpiece pressure can help to support weak or tired chops in order to continue playing. The downside is that too much pressure against the lips prevents proper blood flow, and you wear out even faster - and finally it's game over until you've had plenty of time to rest. The A.P.E. helps players correct this problematic behavior.
It's easy to use - just plug your mouthpiece into it and start buzzing in the lower range. Then increase the pitch of your buzz as if playing a scale.
As you ascend higher up the scale you will naturally start to press the mouthpiece harder against your lips. The harder you press, the more you will compress the spring - which in turn will cause the air outlet to begin closing. If you're pressing too hard, the air outlet closes and the buzz stops because the air cannot escape.
With practice you will learn to ascend higher and higher up the scale without closing off the air escape.