I have now lost count of the number of swimmers who have come to me with issues related to breathing, having been told they should empty their lungs when swimming.
They have generally been told that if they swim with much less air in their lungs this will improve their balance in the water and help bring their hips and legs up.
Let's think about this for a moment: when we are breathing we should be breathing into our diaphragms which is pretty close to the centre of our body. We do not (and definitely should not) breathe into our thoracic cavities (shallow breathing). So air in our lungs (diaphragms) makes us nice and floaty and buoyant. No air in our lungs makes us sinky (not nice). It really is as simple as that.
When cycling or running or playing any other sport we do not empty our lungs. Swimming is no different.
Try this test: Lie in the water face down with your arms in front of you. If you are a bloke the chances are you legs will not be floating at the surface and you may even have your feet pointing down towards the bottom of the pool. Now slowly let the air out of your lungs. Do your hips and legs come up to the surface? Nor do mine.
Of course don't hold your breath when swimming but nice natural easy breathing is the way to go for buoyancy, balance and relaxation.
No air in lungs is uncomfortable and will trigger the amygdala which are the parts of the brain that kick in when we are under threats (like imminent drowning for example). This is not an area of the brain you should be trying to override or a feeling you should try and get used to when swimming.
If your swim coach (yes me included) tells you something that does not seem to make sense, then please ask her or him to explain or demonstrate how it works. Especially if they tell you to empty your lungs.
I am a certified level 3.0 Total Immersion Swim Coach.