At this point in your development, you understand the proper swing plane of the slice. You can repeatedly use good technique, but if you're relying on your arm and hand to power the shot, your slice will never have adequate bite or penetration, and late in a match, when you get tired, it could break down.
So stepping into the slice with your legs and stretching your arms out (with your hitting arm moving toward the contact point and your off arm behind you) will involve your quads, chest, and back—your best power sources.
Besides using your whole body to hit the slice, you also need to have good feel and control. That's where "cupping" the ball begins to become important.
When you cup the ball, you cut under and around the outside edge of it. If you're a right-hander, you're trying to cup the left corner of the ball (the opposite corner for lefties). This helps control the flight path and adds zip to the shot.
If you hit the inside part of the ball, that means your wrist is ahead of the racket face, which will result in more sidespin than backspin on the shot. It takes great talent to control this type of slice because the ball has a tendency to sail. Cupping the outside of the ball is a much more reliable technique.
ADVANCED
Objective: Develop your shot awareness and begin to use your slice as a weapon.
As I've mentioned, slice affords a player variety because there are several different ways in which you can hit it.
At this level, you can call upon all your options, and understanding your positioning and status in the point will help you determine which slice to use.
For example, being on the dead run and far behind the baseline would call for a defensive slice. In an even backhand rally from the baseline, you may choose to use the slice as a change of pace to throw your opponent off. And inside the court, you can move forward and use your slice to attack.
This is what I call shot awareness— having a clear grasp of what's coming at you and what shot to use in response. From this perspective, you can use your slice to create openings by getting your opponent out of position.
For example, if you're in a backhand crosscourt rally and you get a ball that lands short, hit a deep, skidding slice down the line to open up the court. Or from the same position you can carve a short, sharp angle that forces your opponent up and wide into an uncomfortable position--almost like a drop shot, but with more pace and less arc.
If your opponent has to attack off that, you'll have a good look at a passing shot. And if he tries to retreat to the baseline, you'll have an opening on his forehand side. Players with versatile slices, like Roger Federer and Tim Henman, often use this tactic against clay-courters who are leery of coming to net.
During a backhand rally, a sharply angled slice can throw off your opponent's rhythm by drawing him off the baseline and wide of the court.