I once was lucky enough to have an older gentleman demonstrate the yell for me. Clearly he was not a veteran of the war himself, but in his childhood he frequented veteran conventions. He said that the veterans in his town taught the young boys how to do it and that the young boys had great fun sneaking out at night and using it to cause a ruckus.
I wasn't sure if he was doing it correctly, because he was elderly but I remember that it had breaks in the sound, just like in this video. At the reenactments I've been to, the Rebels typically make it a "rolling yell" with no distinct breaks in the sound.
Click the link below to view the video: