The slice is a very fluid motion and does not need to be hit hard. A slice shot is good for placement and keeping the ball low. I use them mainly for approach shots (forehand and backhand) depending on the situation. You will notice that the extremity of the high to low motion is not the same as the topspin's low to high. You do not need to chop at the ball. It is a smooth motion, mainly swinging forward but with a little high to low motion of the racquet. The more "high to low" you hit, the more backspin you create and the lower the ball will bounce. If you hit enough "high to low" the ball may bounce backwards depending on how hard you swing at the ball. Usually this technique is reserved for drop shots. Drop shots are always hit with backspin to keep the ball from getting any closer to the opponent once they bounce and it keeps the ball low giving the opponent less time to get to the ball.
Slice is great weapon to have, especially if your opponent has trouble hitting balls that bounce low. Some players don't bend their knees well when hitting low bouncing balls. Slice also gives you a great way to mix up your shots so your opponent isn't always hitting the ball in the same strike zone. Even if you don't use the slice a lot, it is good to know. A lot of players when running down a ball that is almost out of reach get stretched out. Players with a 2 handed backhand sometimes have to let go with one hand, and this is when slice would be good to use. With the slice you are able to keep the ball lower and hit a better return than just blocking the ball back when stretched out wide. I personally think slice is a great weapon for certain approach shots. If you slice the ball for the approach and the ball bounces very low your opponent must first get to he ball, hit the ball over the net and hit a winner.