Essentially this is a variation of "the Shocker." Swanning occurs when a person places either the pinky and or ring finger into a womans anus while reaching the thumb over the vagina to apply clitoral stimulation. One can imagine the position and reaching of the thumb as the craning of a swans neck.
Woman: My vagina is so sore right now, but I'm kinda horny. Could go for a serious Swanning right now.