No kidding! I'd always imagined it as a sort of repetitive whooping sound that got much of its power from many people doing it at once. But the solo voice in the video was enough to chill my blood. Otherworldly.

Edit: It's worth mentioning/repeating that most historians believe there were rebel yells, not just one style, though they were probably all fairly similar. If that's the case, I think they found a recording of one of the most intimidating variations here.