He lies on his back, she turns her back on him and lowers herself onto his penis in a sitting position. Grasping her legs, she then pulls herself upward so her torso is as straight as possible. She’s supposed to do all the thrusting, but he can help by lifting her at the hips.
From The Perfumed Garden:
TENTH MANNER (Reciprocal Sight of the Posteriors) — The man lying stretched out on his back, the woman sits down upon his member with her back to the man’s face, who presses her sides between his thighs and legs, whilst she places her hands upon the bed as a support for her movements, and lowering her head, her eyes turned towards the buttocks of the man.