Technically Speaking........"How does a Full Face Mask work ?" Does it prevent mouth breathing or does it allow for mouth breathing ?

The full face mask covers your mouth & nose . You can breath any way you want. Mouth or nose or both if thats possible...

Cheers,

Chris

The Flexifit is very comfortable to wear but it is terrible to use!
It has been made cheaply and has no anti-suffacation valve - the ResMed Full Face does have one - the FlexiFit instead has a non-return valve in the elbow, this means that when you exhale you have to FORCE the air out the exhaust vents and this causes a MUCH higher pressure than your machine is running at to exhale against!

I have not tried the Respronics Full Face Mask - but the Resmed one is good for me.