Derived from the actions of the character Patrick Bateman from the film "American Psycho" ...
The act of flexing ones muscles and congratulating oneself in the mirror while having intense sexual intercourse. At some point during the flex off, the flexer must wink and point at himself in the mirror to acknowledge himself as being "Da MAN", even if his partner is not fully aware of the absolute awesomeness that has just rained down upon her.
If there is no mirror nearby, one may still perform the Psycho Flex, though the resulting ego boost may be greatly reduced.
The high from working out an drinking my whey protein had me Psycho Flexing all night with my lady. I think I pulled something.