This is a sexual act usually done by oneself. Ideally, it is done in a desert environment after eating spicy food yet can be done anywhere. After having a bowel movement, wipe some of the fecal matter over your testicles, then lie outside on your back with legs spread. The fecal matter will attract flies which shall land and 'dance' on your testicles thus causes a tingly or arousing sensation.
After I had some really spicy Mexican food, I wiped the other way once and speared my boys, then lied outside with my legs spread a little. The flies came right away and jumped all over it. They gave me the best Mexican Sack Dance ever.