(picture courtesy of earl53 at morguefile.com)
A sure-fire way to cause diver trauma is by shooting to the surface. Thank goodness the divemaster was strong as the student struggled to right himself and keep from floating up to the surface. Neutral buoyancy was a skill to still be learned by the newbie, as was about a jillion other things. But the calm dive master candidate reached over and grabbed the student's BC and dumped the air from the purge valve, all without moving from his spot. Later, Mr. Daisy (the dive master candidate) said, "It's a good thing I have strength in my arms and weight on my side. Otherwise, Student X would have dragged me to the surface with them."
Face it. Most students learn to dive while being over-weighted to keep from shooting to the surface. While in the pool, they learn the proper technique to find neutral buoyancy in their training. First, dump all the air out of your BC while at the surface and hold a normal sized breath. You should hover/float right at eye level and sink when you exhale that breath. But nervousness, full tanks getting lots of heavy breathing, and lack of underwater comfort can later transfer into "fishing bobber" look-alikes. That full tank becomes positively buoyant as it empties!