The German word zwischenzug means intermediate move and it is a common tactic that occurs in almost every game of chess. Picture your opponent making a move that directly threatens one of your pieces. After you opponent has done this you are able to follow up with the zwischenzug tactic. When this tactic is used in the game of chess you will make a move that poses an even more devastating threat, instead of countering a direct threat, which the opponent expected you to do. Often the move that you made will be a direct attack against the opponent's queen or the king. The opponent is forced to counter that threat against his or her queen or king first and this will ideally change the situation to his or her disadvantage.
Just as easily as you can use this tactic, so can your opponent's. Because this is a common, well-known tactic you should always watch out for a zwischenzug. Do not assume that the opponent has to counter your threats immediately, no matter how great they may appear in your eyes. It is good practice to always check whether your opponent has a check or a move that can threaten your queen. Conversely, anticipate your opponent's threats and plan a surprising. Zwischenzug.
Try to use this tactic when it appears to work to your advantage and guard your pieces against it, particularly guard your queen and king against the zwischenzug. This tactic is all about forcing your opponent into making tough decisions. Your opponent has to ask themselves for example "do I take his knight with my bishop, or guard or move my queen?" these types of situations generally will provide you with a tempo advantage as your opponent retreats.