To wipe the palm and fingers of your hand down someone's face, when they are least expecting it.
A perfect Facerape is performed from forehead to chin.
A reverse Facerape (chin to forehead) has (to my knowledge) never been successfully performed (without causing death, of victim)
the girlfriend leans in for a kiss, you pull back and WAMOO; Facerape!
A Friend concentrating way to hard on his game of Call of Duty = perfect Facerape opportunity.
Fun for all ages.