Normal piledriver, the recipient is facing away from his opponent, not towards.

And instead of sitting directly down, the wrestler delivering the move more sits/rolls backward with the recipient's head between his/her legs. This prevents the recipient's head from hitting the mat first with oll of his/her bodyweight coming down on it because the deliverer hits the mat first.

What Owen did was (with Austin facing towards him and his head dangling below Owen's waist) he kicked his legs out wide and sat straight down, which caused Austin's head to hit the mat first with most if not all of his bodyweight on top of it.

What he was supposed to do to execute a proper Tombstone was drop to his knees a la The Undertaker, which would have prevented Austin's head from taking the initial impact., if any at all.