Most coaches stress that you should place most shots to one of three spots: the wide backhand, the wide forehand, and the middle. The middle is roughly the opponent's playing elbow, the transition point between forehand and backhand. While most players can cover the wide corners reasonably well (unless drawn out of position first), the middle is often far more difficult to cover, even if in position. It also draws the player out of position, often leaving one of the corners open. While it often takes two shots to the corners to be effective (the first shot is mostly to get the player out of position), shots to the middle are effective on the first shot, and often leave the player open to all three spots on the next shot.
So how does a player cover these middle shots, and recover for the next shot?
How you cover the middle depends on your playing style, but there is a general guideline for most players: If you are rushed or close to the table, favor your backhand. If you have time or are off the table, favor your forehand. (This is especially true if you have a good forehand loop.) But remember these are guidelines. For example, if you have a much stronger backhand, use that whenever possible to cover the middle, and then try to dominate the rest of rally with your backhand. If you have a much stronger forehand, then perhaps play a step off the table so you aren't rushed, and use that cover the middle, and then try to dominate the rest of the rally with your forehand.
But it's not enough to just cover the middle; you have to be ready for the next shot. There are two possibilities: recovering after a backhand, and recovering after a forehand.
If you play a backhand from the middle, then you leave your wide backhand open. If you are relatively close the table, then you have little time to recover. So the focus here is to follow up by rapidly getting back into position to cover that wide backhand. If you have a strong backhand, then you can take advantage of this situation by often getting two strong backhands in - the first from the middle, and by playing it wide to the opponent's backhand, you likely get a crosscourt return to your backhand, and so get to play a second strong backhand. (In fact, you can keep doing this, locking up the opponent in a backhand-to-backhand duel, if your backhand is stronger. But don't just go to the backhand - also attack the middle, and when you see an opening, or if the opponent is weak on that side, the wide forehand.)
But here's the problem - after hitting a backhand from the middle, if you rush too quickly to cover against a shot to the wide backhand, you might inadvertently leave the forehand side open, and a smart player will follow his shot to your backhand with an attack or quick shot there So your best response after your backhand from the middle is to move back to cover for the backhand, but come to a stop before the opponent hits his next shot. It's better to be slightly out of position but in a ready stance as the opponent hits his next shot than moving into position as he does so.
If you go to your opponent's middle, and he covers with a backhand, then watch to see how he recovers. Most often he'll be open on the next shot on one of the wide corners, and it is your job to see if he leaves the wide backhand open, or covers it so quickly he leaves the wide forehand open. If not sure, there's a simple solution - go to the middle again!
If you play a forehand from the middle, then you leave the wide forehand open. But if you are at least a step or two off the table, you have time to cover for it. In fact, if you are a good forehand player with good footwork, then you very much want to play forehands from the middle of the table, since you really get two for the price of one since you will likely get a second forehand, and you likely can end the point with one of these forehands.
But this often depends on the strength and (more importantly) the depth of your first forehand from the middle. If it is weak or lands short, then it is easy for your opponent attack aggressively, and in this case your wide forehand is often open. But just as with the backhand, if you try to cover that side too quickly, you'll leave the wide backhand open. So again, come to a stop and get into a ready position before the opponent hits the ball.
If you go to your opponent's middle, and he covers with a forehand, then watch to see how he recovers. Most often he'll be open on the next shot on one of the wide corners, and it is your job to see if he leaves the wide forehand open, or covers it so quickly he leaves the wide backhand open. If not sure, just as before there's a simple solution - go to the middle again! But the danger here is you may let a forehand-dominant player play his forehand over and over from the middle, which is almost always a mistake. If the opponent is looking to play a second forehand from the middle, then go after the corners.
It's a cat and mouse game, where each player is trying to get the initiative, either by attacking the middle or attacking from the middle. If one player can draw the opponent out of position by going to the middle, and then attack an open wide corner, then he's won this cat and mouse game. If the other player makes a strong shot from the middle and follows it with another, then he has won.