When a genius idea strikes you in the middle of a huge dilemma, saving time, effort, money, you and friends from sobriety or ticketing/over-ticketing/arrest, etc...
Opposite of brain fart, cerebral blockage, bad idea, and so on.
Fred: "I can't believe this store stops selling beer at 9 pm. What are we going to do?"
You: "Brain wave! The one down on Jeff Davis Hwy sells until 11! Get in the truck and let's roll!"
Fred: "You are a genius!"
Bob: "My car stopped working, how are we going to get to the show?"
You (again!): Brain wave! My neighbor always leaves the keys in his ignition. Let's roll!"
Bob: "You are a genius!"
A brain wave is a stupid thought that some kind of idiot gets after a sleepless night. This brain wave can become a brain Tsunami when this idiot is some kind of boss.
A brain farth is also some kind of idea.
Good examples of brain waves can be found in politic environments, for instance: "What about attacking Iran with nuclear bombs ?"