Undulation (Body Roll)
To do an undulation you should start with a vertical rib circle and the tuck and release of the pelvis. Let's start with the rib circle by pushing the ribs forward, up, back and center. Then round it out till it's smooth(This is what your torso will be doing). Next we'll work on the pelvis. Start with your feet parallel and facing forward with your knees slightly bent. Now you tuck, tuck the tailbone under causing your hips to go forward. Now, release it back to center and the push it slightly back and then back to center( That's about all they do). Now we combine them. Push the ribs forward and the pelvis back. Next, push the ribs up and the pelvis goes back to center. Then, pull the ribs back and tuck the tailbone under you. Finally, you return the ribs and hips to center. Try to smooth it out. Also, try it with one foot in in front of the other or on the balls of your feet(helpful for walking). Make sure your roll down the spine though every vertabret.
Reverse Undulation (Body Wave)
The reverse undulation is done by rolling up the spine as opposed to rolling down the spine. First bend your knees and keep the feet parallel and facing forward. Now, tuck the tailbone under causing the hips to move forward. Next, push the abdomen forward and release the pelvis back to center. Then push the chest forward and tilt the pelvis back slightly. This movement is often used to accentuate a sharp change in the music, by snapping the rib cage down once you finish rolling up. This movement is not usually done repeatedly, one after another. It may help you to think of stacking one vertabret on top of another, one at a time.
Structural Roll
I like to think of this move as a more tribal undulation. You start by putting your right foot in front of you and the left bends in back. You step forward on your right foot and you lift your chest. Now lean back on your left while rolling down the spine. Tuck the pelvis under you when you reach the bottom of the spine. Unlike most movements there is not a lot of effort to keep yourself level, you may move up and down with the movement.
