Creo necessitas "Vault". And to use it when the enemy is downed. (Keeping the enemy downed does not presently work properly afaik, so this options out as of now).
Another way, if I'm not mistaken, is to be grounded on your back against an aroused harpy. It's been a while since I've tried it, so don't hold me to it, but I think the harpy once intiated 69 with me while I was in this stance.