The first time you see people dancing zouk you have the feeling the dance is very fluent. The step of today is a perfect example: the wave!
For the wave you have to learn to isolate your chest, bringing your chest to the front and back.
For the ladies: the most important thing to understand that the guy is leading the wave, is keeping the contact with the hand on your back. You have to be connected with the guy.
For the guys: at the beginning is difficult to coordinate your body. You go to the front with your shoulders and your chest transfering the weight to the front (your left foot) and at the same time you lead the lady to you.
One advice: don’t think, you are going down while doing the wave.