In this youth softball drill, Jennie Finch explains that the keys to throwing a screwball are grip and positioning on the softball mound.
Let me show you how I throw my screwball. To get the grip, for me, I find the "C", put my four fingers across that "C". When I come veer on my screwball, I step a little bit to my left, allowing me to get inside the ball, trying to pull the ball here almost like pulling a merry-go-round. On the finish, I want to finish by my left pocket, creating the spin, trying to create that motion into a right-headed hitter.
Let me show you how I throw my screwball.