There's also the head bobble, common in south India, which looks a lot like an ongoing head shake but generally means something like ongoing agreement.
Referring to it as "ongoing agreement" reminds me of something I think of as the "confessional no." When addicts talk about their addiction, there's often a rhythmic shaking of the head from side to side like a continual "no," but that speeds up and slows down with some relationship to the grammar of the sentences used.
You can probably make yourself do it by saying out loud: "When I was young, I thought I understood everything." It's like an ongoing disagreement.
https://en.wikipedia.org/wiki/Head_bobble