I personally observed this event only once ....
on one morning, some years ago, I had planned an SM session with my slave caroline and had invited one of my best friends to join me in submitting her.
We had her naked and borught her up in the barn, where we placed her on a low table, on her elbows and knees, legs apart, head down and ass lifted up, ready to receive a good whipping and our cocks soon after.
Well, I heard the sound of a ritmic dripping of water on a wooden surface: I found out immediately that there were actual drops of wetness dripping form her well lifted pussy on to the wooden surface behind her...
I immediatley drew the attention of my friend to these regards, using quite trivial expressions and he replied with some very rude remark about our slave, who flushed in humiliation while, to her dismail, the dripping only increased in frequency

As you can imagine, our plays were particularly hot that morning.