Here you can see the air outlet hole and end of the mouthpiece inserted into the A.P.E.:

 

Spring not compressed and the air outlet is wide open. No problem buzzing in this position.

 

 

Spring slightly compressed and the air outlet is staring to close. You will start feeling more back pressure in this position.

 

 

Spring is compressed even further by more mouthpiece pressure and the air outlet is almost closed. 

Buzzing is starting to get very difficult because of the back pressure.

 

Too much mouthpiece pressure has compressed the spring enough that the air outlet is closed. 

Buzzing cannot continue until you reduce the mouthpiece pressure enough to open the air outlet hole.

 

Ken Titmus demonstrates the plastic model:

In this sequence of photos you will notice Ken is pressing harder in the second photo, and has 

completely closed off the air outlet in the third photo by applying too much pressure

 

 

 Notice!!

If out of stock, it will be automatically backordered.