spin the bull-roarer in a manner that doesn't hurt yourself or the people/things around you. See the video to get a feel for velocity.
"Ever since prehistoric times the bull-roarer has been a symbol of fertility and evidence of them has been found in paleololithic sites. The sound of the bull-roarer is said to be the voice of an ancestor, a spirit, or a deity. This is very important because this plays a role in certain rites of passage in some areas of the world." (http://www.music.vt.edu/musicdictionary/textb/Bull-roarer.html)
Bull-Roarers are serious business to some folks; ever seen crocodile dundee?
make a "genuine sounding" bull-roarer out of a ruler and sturdy string.