This is a unique gesture, the wobble bobble. Each race on Earth has their own special gestures and behaviors. The Chinese tend to keep a distance from others. The westerners often hug acquaintances as some sort of greeting. The Indians, well, when they talk, they tend to shake their heads from side to side and also up and down.
This Indian wobbling is not considered a nod or a shake. Therefore, I once asked my Indian lecturer who was from India. I asked him something and he wobbled his head. I did not know what he meant but in the end, I figured that he said yes. I thought of the time when I asked him something and he said no. He shook his head and said "no" firmly. Therefore, the wobbling should be Yes.
This wobble bobble is unique to the Indian race, especially the Indians who are from India, and other non-Indians might have communication troubles when conversing with them.