Kumbha is a pot and Purna Kumbha means full pot. Kumbha is actually a description of what we call in yoga as Kumbhak meaning when your belly is filled with air. From our point of view Purna Kumbha is when the belly is completely filled with air and if you are a real Kriya Yogi you do not have the need to exhale. But don't try that out, you might damage your lungs!