A shake of the head from left to right in a 45 degree angle which usually means a "Maybe", i.e. Neither a Yes nor a No.
Typically used in India and nearby regions to say "yes" out of politeness or duress, but really meaning a "maybe".
1. There are times when the Indian headshake can be useful. Like when your girlfriend or wife asks you whether she has gained weight.
2. Seagull Manager: Can you get this presentation done by this evening...
Junior Employee (fearing for his job): <does the Indian headshake> I think so
Seagull Manager: Great, I'll expect it in my email by 4pm.