The shuutobooru or "shootball" (but almost always called by its shorter name, shuuto) is a pitch thrown by Japanese pitchers that breaks down and in on right-handed batters when thrown by a right-handed pitcher, somewhat similar to a screwball, although with less break. The shuuto is often described in English as a "reverse slider," but this is not strictly the case, as the shuuto generally has more velocity and less break than a slider.
The shuuto begins as a fastball, taking a straight path toward the plate. However, the pitcher has put a slight spin on the ball, such that as the ball's velocity decreases, it "rolls over" and drifts back toward the batter.
American announcers would probably classify the shuuto as a "sinker" if they saw one, but the American term "sinker" encompasses a much wider variety of pitches, of which the shuuto is a very specific type.