Brain is NOT another word for head like many people seem to think, but rather the art of a girl sucking and massaging the male testes with her mouth and tongue.
i have never had the fortune of recieving brain, but i hear it is quite the superb experience.
when after jacking off for a long time or having sex for a long time and you finally cum and the load that you blow is so immense that you can feel your sack emptying out leaving the skin of it so tight that it constricts around your balls leaving your sack looking like a brain.
Dude i sobered up halfway through and saw how not hot she was and it ended up taking me so long to finish i definitely had some brain drain.
driver: Hey bro, throw some music in, I'm tired of this shit.
Passenger: Umm.. let's see...
Driver: (5 minutes later) Dude, don't brainfuck it, just grab a CD and throw it in.