1)Most commonly used when referring to a woman who is down for the gang bang with you and your buddies.
2)Or who is completely down for you filming your sexual relations without having to hide the camera.
3)It can also be used when in reference to questioning someone new to your circle of friends integrity or loyalty after committing a crime.
Mike: " Dude, call Heather over here, she is D.F.T.C." ( down for the cause)."
Joe: " Hell yeah my dude, she is always down for the gang bang!"
Billy: " Mary, can I film us having sex?"
Mary : " Of course Billy, you know I'm always D.F.T.C ( down for the cause).
3) John: " Dude, do you think Dave is going to snitch on us for stealing that car, we just met that dude?"
Steve: " No bro, he's cool, he's D.F.T.C.(down for the cause) he won't dime us out"