The Butterfly position is extraordinary for the edge of the bed, sofa, or almost whatsoever border for that matter! To come in this posture, the women lays back on nearly any edge sufficiently broad and often sets her feet on the floor, whereas the penetrating mate kneels or stands between her legs. This lovemaking position is really comfy for both mates, although a pillow or some cushions underneath the knees may be practical for a kneeling penetrator.
The original, and more difficult ariant of this position is more stimulating for the man hits the woman's g-point every time. The lays back on the bed, and the man stands in front of her. She places her feet on his shoulders, and he lifts her buttocks so that only her head and shoulders touches the bed. See second picture below.