Here is more information to help with Relaxed Diaphragm Breathing questions.

 

 

Related Topics are:
Hyperventilation
Breath Hold Diving
Shallow Water Blackout

The Problem with Hyperventilating before a Breath Hold dive is that Carbon Dioxide levels might fall so low to prevent the “urge” to breathe.  This would of course cause a fall in the Partial Pressure of Oxygen (Hypoxia)
As the Skin Diver ascends and the surrounding pressure decreases, the Partial Pressures of Oxygen will fall even lower causing the diver to loose consciousness as he nears the surface. This is called Shallow Water Blackout.

 

Here is a snippet from the current PADI Instructor Manual. Here you can see that it gives guidance to Instructors when teaching this topic.

Breath Hold Diving

Airway Control