This is the link where I found the information: http://www.heartsart.com/info/innewsdetail.asp?nid=5&ID=114
Essentially, you lay down some paint and while it's still wet you sprinkle baking soda over portions of it where you don't want the paint to stick. Then when it's dry you rub off the baking powder and voila!
On my first attempt I painted a canvas with one layer of yellow and then sprinkled the baking powder on. I was hoping to get some of the white canvas to show through but as you can see from this picture, the baking soda didn't have much of an effect (I had to put a stamp on the canvas to give my camera something to focus on):
It occurred to me that perhaps I was being too timid with the amount of baking soda, so I tried it again. I sealed the yellow paint layer with some gel medium and then put a layer of green paint over it. While the green was still wet I put baking soda all over it. I used a LOT more this time as you can see from the picture below:
Once it dried and I rubbed off the baking soda I was left with this:
I don't love the result, but I plan to do more layers and keep working with it this weekend. I'll post more pictures in a few days.