I was sounded at my very second session... didn't think of this during the pre session talk. i only gave her admission for trying some electric. And laying spread eagled and cuffed on the bondage table, i was in no position to negotiate... and it was just ok. No big thing, just some curious feelings and a strange vision seeing her penetrate my 'hole'. So, it's not a limit, but not an preferred action in sessions. Later, i experienced this in combination with current - that's a totally different story...