In the video, the first girl interviewed is shaking her head "no" while talking. I see that often in people being interviewed like this. It's just extra obvious here. Another typical example is someone being interviewed like "we're investing a lot in this project in our school which is great for the students [shakes head no while saying so]" or e.g. a kickstarter video like "this product is going to revolutionize the ..... [shakes head no while mellow guitar music plays in the background]"

What does the no shake mean? Does it mean what they're telling is not true? Or is there some other reason why people instinctively shake no while talking in interviews?