Coined by Te'Devan Kurzweil, mindflanking is the art of saying something that nobody expects and expanding their mind in doing so.
Where a generally common/idiomatic exchange may be occuring, you instead say something totally whacked out and unexpected. You break a social convention and their whole perception of that convention is altered.
I'm not sure about the validity of the perception altering, but it's fun.
Man 2: I'd like to sleep with my second cousin.
Damn...you've been mindflanked.