In my third black and white painting, I tried placing the canvas on my studio floor and pouring thin black paint onto it. That gave me confidence to do more pouring in this fourth painting. Since it already had a well-organized background to play against, I placed it on the floor and poured a thin white paint onto it.
After the white paint dried, I then poured a thin black paint. The trick in both pourings was to have the paint thin enough to pour, but thick enough to be opaque. Then, I had to move my hand in the right motions to get the general swirling shapes I was looking for, as well as some small splashes.
It takes some practice, and a lot can go wrong, but I was happy with the outcome of this "accidental" process.