Who it's for, and when
For couples of women, first of all. The position comes from love between women: two bodies meeting vulva to vulva, and a pleasure that's mutual, each woman feeling the friction on her clitoris and inner labia. With or without a toy, as you please (see the variations).
For man-woman couples too. She lies on her back, he lies across her, and you start with caresses, genitals pressed together. Penetration comes later, gently: his penis enters at an unusual, slightly twisted angle, so ease in with small movements, and if anything pulls, slow down or change position.
For partners who know each other well. This position calls for a good knowledge of each other's body and a solid intimacy, physical as well as emotional: it's a position for partners in crime more than for a first night.
When? On evenings when you want to make love without penetration, or to play with angles and positions over the course of the same night. And at those moments when you want to let go without having to look at each other.
Precautions: get the whole area nice and wet before you start rubbing. If the friction or the penetration hurts, stop, and if the pain persists, talk to a doctor or a sex therapist. If you're pregnant, have a painful back, hips or knees, or limited mobility, ask a health professional for advice first.