Hitting the ball behind your body
Sometimes you find yourself in a situation where you arrive very late to hit the ball. Either the ball has already traveled past you, or it it close to a second bounce on the ground. In both situations, you won't be able to save the situation with your regular technique.
Let's take situation #1 first: You opponent has hit a dropshot, and you are forced to literally scratch the ball off the ground before it bounces twice. Now you need to get your racket under the ball and lift the ball over the net any way you can. The problem is that with your normal grip you will have problems bringing the racket into the correct position under the ball. What you need to do now is to use a forehand grip for the backhand, and a backhand grip for the forehand shot! That way, you can slide the racket under the ball, the angle of the racket allows you to hit the ball upwards and over the net.
In situation #2, your opponent has hit a shot deep into the back of the court, and because it has already passed you, you cannot hit it in front of your body. Because of the fact that now the point where you make contact to the ball has changed, your grip has to change too. Again, use the backhand grip for the forehand and vice versa! See how with the inverse grips, you can hit a forehand slice and backhand slice shot as a last resort. When you hit behind the body, you cannot give any body rotation to the shot. So what you need now is pure racket rotation, and the only way to do that now is from your wrist. Hit the ball rotating from the wrist and give it a lot of backspin.
This is a very tricky shot, and it will not always go into the court. Do not hit that ball too fast or it will go out. In this situation, all you can hope is for the ball to go in somehow, so that you can stay in the game.