Holding your breath while diving is a pretty sensitive subject. Gas Embolisms are a leading cause of serious injury in the DAN reports. Most of these are caused by quick ascents to the surface while holding your breath. Many agencies have noticed this, and try to ingrain in their students to never hold their breath. This is good because it makes students more conscious of their breathing and when they are holding their breath. The downside, however, is that it paints a somewhat unrealistic picture about just how dangerous holding you breath is.
For example: When breathing normally, I typically inhale slowly, hold that breath for 4-5 seconds, and then slowly exhale. This sort of slow and relaxed breathing rate is actually good for diving, and encourages good gas exchange in the lungs. As a matter of practicality, it also allows very precise adjustment of buoyancy, and even allows a diver to rise or fall in the water column just by controlling their breathing. It is something I've noticed all divers with good buoyancy do...but technically it is breath holding.
Problems don't really arise with breath holding until you lungs are full, and the airway is closed. Imagine you are about to dive into a pool, and try to swim to the other side underwater. You are going to take a very deep breath--to the point that you can feel your lungs expanding against your chest. When you inhale this deeply, your lungs are all stretched out, and you have to close off your airway to keep them from deflating like a balloon. This is the sort of scenario you need to avoid when diving; you want to avoid closing your airway, and you want to avoid very deep breathing.
So to answer the question about buddy breathing while ascending, the real key is to take a normal, relaxed breath. If you can manage to take a nice, relaxed breath while buddy breathing, you should be able to hold it in you lungs for a couple seconds WITHOUT clenching down on your airway, and slowly release your breath over 5-10 seconds before getting the reg back, and getting your next breath. Yes, you will rise a couple feet while you start exhaling, and yes, this is perfectly safe. The key is to remain relaxed at all times, and never clench down on your airway.
HTH,
Tom
PS, in order to do this, you do need a fair amount of control over your buoyancy, and you need to be relaxed in the water. These are two things that a lot of divers lose in an emergency, which is why a lot of divers have gotten in trouble during buddy breathing ascents.
Practice is one thing, but in reality, I've never heard of anyone having to share air for any reason other than they were an air hog with just a couple of exceptions. I did read about tank oxidation goo that killed a couple first stages in Coz... yikes!
