I have no research or data to support this but my dive instructor stated during training one time that oral inflation during the dive actually wastes more air than manual inflation because of the way it changes your breathing patterns when you do it. So you save some air in your tank by orally inflating but then make up for it by breathing harder after you do so.
Don't flame me. I'm just the messenger.
HM
I would say that depends on the individual.. new OW student for sure. Competent, non stressed experienced diver with a good breathing rate.. not so likely.
As with everything else There are no absolutes.. works for some very well.. others too stressed or just not comfortable with it.. not so well. I just think it is a waste to use tank air for predive and post dive inflation and if you are comfortable using manual inflate under that water and it maintains an important skill why not do it?
Some find it easier to fine tune that way and many places in the world it is illegal to spear on Scuba or take "bugs" (certainly the case everywhere I have ever dived) and not everyone is overloaded with a Camera so the task loading thing becomes a non issue especially when one has done it enough that it is second nature. Just my .02