When someone starts to hit on someone else. They also call it giggity faffing.
"Hey, dude I'm gonna start talking to that chick over there; she seems fine."
"Ooooh some diggity faff!"
"Yep, I'm doing some diggity faffing!"
*guy is talking to girl they like, and someone else walks past and shouts "diggity faff" at them - for trying to get it on with the girl."