Blokh’s The Art of Combination, unfortunately out of print at this writing, is a fine book on chess tactics. At the beginning of each chapter it presents a few elementary examples of the pattern that is the subject of the problems that follow. The examples are abstracted; all that is shown is one corner of the chessboard where all the relevant pieces reside. The technique is a useful one for presenting the basic idea behind the use of interference to loosen a mating square (or to win material by threatening mate), so we will begin here with a few of these odd-looking examples that Blokh suggests.

In this first example you see that White has a queen and rook aimed at f8 and would mate there were it not for the protection provided by Black’s queen. The Black queen can't be taken or driven off with a threat, but White looks at the line running from the queen toward the mating square and sees that he can land a piece between them with Nf6+. When you interpose a piece like this, by assumption it can be taken by the piece you are trying to block; it therefore needs protection, which the knight here receives from the pawn on g5. If Black plays QxN, he loses his queen and is mated soon; but if he plays anything else he is mated even sooner.