When two people are doing a 69, the person on top takes a shit on the face of the person on bottom. Then the top person leans back, sits on the bottom's face, and moves back and forth to rub the shit all over their face. When the bottom gets up, their face is covered in black, just like a coal miner!
Susie was tired of her boyfriend having all the fun with moves like the bucking bronco and the dirty sanchez. She cooked up a move she could surprise her man with -- the coal miner!
A coal miner is a sexual term that relates to a type of woman with an amazing body but a not so great face. This is a type of woman one would like to have sex with in the dark but wear a COAL MINER's helmet to stare at her ass doggystyle.
Guy1: damn that girl has a nice body
Guy2: eh but look at her face....
Guy1: She's definitely a coal miner