Sometimes, when submerged, you need a quick way to get your buddy's attention. When your buddy is fiddling with his gauges - oblivious to the school of manta rays soaring overhead - you need a gentle way to say, "Hey buddy look over here!" Since flailing your arms and hollering into your mask won't accomplish anything those are the situations where you need a Sub Duck.
The sub duck is so called because it sounds like a "quack", the sub duck attaches to your BCD inflator hose. The sound carries for a long distance underwater; one little blast of air is all it takes to get everyone's attention and to signal your fellow divers that there is danger or something to sightsee.
The sub duck is one of several devices and gadgets where its only purpose is to make a little noise. A tank banger is probably the simplest. A tank banger is a large metal bead or nut threaded on a bungee cord around your tank; to alert your fellow divers you need only snap the cord against your tank to produce an audible underwater signal.