I was trained to do this in grade school, again in basic rescue, and again for SCUBA certification. In grade school the consensus was that the gym teachers whipped this out when we were being too obnoxious for them to want to hear us speak.

Boring beats the heck out of drowning.

Before you test it make sure that the people around you know what you're up to, or you might end up getting rescued when you don't need it.