Usually, my female Newfoundland will part her tail with her rump facing me, look back at me, and even sometimes back into me. She does sometimes do this on her own, especially in heat. But most of the time I'll be petting and rubbing her lower back and she'll start to do this. A lot of times I'm not even trying to engage her in sex, her lower back and her chest are her two favorite spots to be pet. But I guess a nice rub-a-dub-dub feels relaxing and sensual to her. When she's horny she lets me know!