The Landslide sex position is a challenge to get into.
She lies down on her stomach, propped up on forearms and with legs straight and slightly apart. He sits right behind with his legs in front of him and his hands on either side of his body for support. He leans back at a 45-degree angle to her body so he can penetrate her from behind.
As he rocks forward and back, she brings her legs together for a tight fit.