Slide Step vs. Knee Lift
A slide step toward home involves the lead foot being lifted a few inches instead of the high knee lift when delivering a pitch from the set position. Developing a good slide step is difficult for a lot of young pitchers and probably isn't needed in the youngest leagues. If a pitcher wants to use a slide step, he must practice it along with his other motion. There is also a tendency to rush toward the plate when using the slide step and throwing the ball before the pre-release mechanics have all had a chance to take place. Besides the loss of accuracy, this can obviously injure an arm.
A slide step isn't needed on every pitch when watching a man on first. In fact, one strategy involves using a knee lift on the first pitch followed by a slide step on the second (at least early in a game). The logic is that most base runners won't steal on the first pitch and after seeing a knee lift they are more likely to go on the second pitch.