When a woman is lying on her neck, hips up and a dude is standing up squatting and thrusting into her. Most porn stars can't do the pile driver for long because they pass out when too much blood rushes to their head.

For most references it is likely a melding of 1 and 2.  The woman does not necessarily have to be lying on her neck, nor does she have to have her legs wrapped around a guy's waist.

The key is that the guy is positioned higher than the woman while the woman's pussy is presented in such a manner that entry is almost perpendicular to the floor as opposed to parallel to the floor.  The resulting action is that the guy's thrusts are downward into the woman.  One of may fav ways of doing this is when a woman lies on the bed as I stand along the edge of the bed.  I lift her ankles up to my shoulders but she can still have her neck, shoulders and upper back lying down.  Her hips and pelvic area are raised to just below my hip level and I thrust while standing (although stooped slightly).  The visual is fantastic, the entry is balls-deep and it's a great f, although the lady does need to be free of upper back problems.

...Dude, you posted the question and the link that gives you the answer.  So now I'm trying to figure out who's more stupid?  You for asking or me for answering?  I fear it's me. LOL

Usually because you left the seat up.

I think I saw that hottie Randy Orton use it on RAW.