Girls lying on their stomachs with their feet up in the air. Although the title of this subreddit is Stomach Down Feet Up, the general rule for acceptable positions is that the girl should have her knees on the ground and her feet in the air; with her stomach, chest, or elbows touching the ground.

Olivia Munn
By the pool
Hilary Duff
Blanc noir
Stomach Down Feet Up Blowjob - Full video in comments ;)