Coincidentally, I just read something about breathing on this coaching site:
http://www.flammerouge.je/factsheets/breathing.htm
You can skip down towards performance breathing if you like.
What he describes sounds similar to this device but you can do it with your tongue for free.
One you may want to try is my (although I’m sure I never invented it) tongue/valve technique. I’ve done a bit with aero and fluid dynamics and know a bit about crowd control and emergency situations; I have an eclectic CV. I joined the two bits together and came up with this system, which works for me.
When you breath out, stick the tip of your tongue at the base of your bottom set of teeth, and exhale from the diaphragm as described above. Get it all out smoothly and calmly.
When you breath in, through your nose and mouth, put the tip of your tongue behind your top set of teeth. Whoever said “that causes an obstruction” go to the back of the class! What it causes is a low pressure area behind the tongue.