Forcing Moves
Since there are on average about forty or more possible moves in most chess positions, looking ahead even one move becomes very difficult if done randomly. Trying to look two or three moves ahead in this fashion is virtually impossible: to each of my forty possible moves my opponent has forty possible replies. That comes out to 1,600 possible moves. And that is looking exactly one move ahead.
This is fine for a brute force computer, but impossible for a human brain. Fortunately, this sort of number crunching is not necessary or even desirable in the pursuit of good chess play.
In order to drastically cut down on the number of moves we need to look at in predicting the near future of a game, we need to find a device that limits our opponent's possible replies. That device is the use of forcing moves.
A forcing move is a move that forces our opponent to respond. An example of a forcing move that you already know about is check. When the enemy is in check, his possible replies are limited to the available ways to get out of check.
Manageable Look-Ahead
By using forcing moves you put your opponent in a quandary. He no longer has the usual forty or so moves to choose from. He must restrict himself to the available moves that get him out of whatever fix you have put him in. That may be as little as two or three moves he will have to choose between. That will certainly make preparing your reply to either of two or three moves by your opponent more possible! Keeping five to ten possible moves in mind may not be easy, but it sure is a lot more possible than trying to keep up with 500 to 1,000 possible moves!
If your opponent isn't very experienced, he might not know that your move was forcing. In that case, you won't do so well in predicting his moves, but that will present no special problem because you were counting on good moves from your opponent. If a poor move comes in its place, all the better.
Other Threats
A check is a direct threat to the king. Any move that threatens to capture a piece or a pawn next turn can have a similar effect. If your opponent does not want to lose that piece or pawn, she will have to meet the threat in some way.
White threatens to capture the undefended bishop on b4.
A threat to promote a pawn is also a great way to cut down on your opponent's possible replies. Since a pawn promotion generally means trading a pawn for a queen, this can usually not be ignored.
There are still other threats that, while not so dramatic as threats to capture or promote, can still be used to cut down on your opponent's possibilities. These include threats to double or isolate the enemy pawns or a threat to get a knight to a fine outpost square. You can threaten to control the center or to bring more pieces in play than your opponent has available.
Recognizing threats is essential to good chess play. Whether it is a queen or a key square under fire or the possibility of getting one of your pieces tied up in defense, you can't do anything about a threat if you don't know it is there.
White threatens to destroy the Black king's protection by 1. Bxf6.
Anything that helps your position or hurts your opponent's position or both is generally worth threatening.
