Belly Down
When she’s on bottom, lying face-down on the bed while you lie down on top of her (or hover above her in a semi-pushup position, entering from behind. Her legs should be straight and her hips slightly raised to meet you.
She likes to be belly down when she's railed.
belly down date
A date during which a male expects to get a lot of sex.
"Hey, Todd! I here you have a date tonight. What's planned?" Todd responds: "Nothing specific, but I hope it's a belly down date!"