A sexual act where the man is getting his penis stroked or sucked by a woman. As he gets harder and harder, the woman then punches his balls sharply along the bottom, causing immediate and violent ejaculation. The ejaculate may go flying several feet in the air, if it isn't being covered by the woman's mouth.
The "Asian" part comes from a well-known porno video, depicting an Asian woman doing this to an unseen male's scrotum. It doesn't really require that an Asian woman do it. Nor does the partner even have to be a woman for that matter.
You know, my girl Maya, was sucking on me one day like normal, but then she stopped, looked at me straight in the eye, and just Asian Ball Punched me. My cum went flying into her hair. Then she punched me again, and it went flying through the air and landing on the wall across the room.