It is when you are having sex in the "pile driver" position. The man applying the "pile driving" then pulls out to cum all down the face and tits of the person that is upside down in the pile driver receiver position. Because the receiver is in the upside down position, all the cum that shoots onto her tits, will still drip down onto her face.
I got so excited pile driving my girlfriend while she was upside down with her ass and pussy in the air, that I pulled out to cum, and all the cum sprayed downward all over her face and tits. The "pile driver money shot" makes the best cum on face.