The ball travels with a lot of velocity, but with a tumbling kind of rotation. The rotation slows down as the ball approaches the plate, and if delivered correctly, the bottom kind of falls out of it. The hard slider or short curve, as I used to call it, has a certain amount of lateral break and a certain amount of down break. The harder you throw a slider, the shorter and quicker the break you can get on it. The release technique is between a curve and a fastball.
Some pitchers release the ball off their middle finger.
I throw my slider off my index finger. I try to feel like I'm wiping over the outside of the ball as I snap it, in order to give it some backspin and sidespin. Other variations Also, a lot of pitchers today throw a slurve. They pitch the slider as if they are throwing a curve, and the ball comes out in a big, sweeping flat curve. I consider this pitch to be just a rather sloppy slider.