An arse valley wank...(not to be mistaken for anal sex..it's similar to a tit wank).
The woman lies on her front while her guy sits on the small of her back facing her feet. He pops his penis between her bottom cheeks
and squeezes them together...and off he goes!
Sam was concerned when Danny asked her for a Backons...but after he'd explained that it wouldn't make her eyes water, she agreed...and actually found it quite pleasurable (she was even able to finish her crossword).