Suppose you see an enemy piece or square that you want, but it has protection. Your options, as we have seen, include capturing the guard, threatening it, or going after something else it protects. They also include this final idea: blocking the line between the guard and the piece or square it is trying to protect. You may be able to do this by planting one of your pieces between them, or by making a threat that causes your opponent to put one of his pieces between them. This motif is useful to understand, but since several conditions are needed to make it work it does not arise as often as the others. We will not spend as much time on it as we have on the other methods of removing the guard.
Like much of chess, interference is all about lines.
The first order of business in the study to the left is to note your possible captures, and in this case Black has just one: his queen attacks White’s rook. The rook is guarded by its queen; can the protection be removed? There is no way to capture White’s queen or drive it off with a threat or take anything it protects. But look at the diagonal between the queen and rook. Might Black obstruct it? Yes, he has a piece that can get there—and with check. He plays Ne4+. White’s natural response to such an obstruction would be to take it with his queen, but the knight has protection from the pawn on d5. So White has to respond to the check by moving his king, and Black takes the rook with his queen next move. Note the importance of the check Black’s knight gave when it moved. Without that threat Black’s move doesn't force White’s reply, and thus leaves him a move to just relocate his rook someplace safer.