Sometimes when the middle piece in a skewer jumps out of the way of your attacker, it can move to a square where it protects the piece behind it—the piece you were hoping to take. This isn't necessarily fatal to the skewer’s success; we have seen cases where the skewering piece is a bishop and it can turn a profit by taking a queen or rook regardless of whether they have protection. But a skewer delivered by a queen usually will be worthwhile only when the piece targeted by it is loose. In those cases an otherwise fine skewer may be ruined if the middle piece can move to protect the rear one, so we need techniques to prevent this from occurring. They are considered here.
This first position illustrates the problem in insoluble form. Black’s rooks are arranged in a pattern that should be immediately recognizable as a setup for a skewer. White’s queen can run through them by moving to c4. Is the skewer sound? No, because when one rook moves out of the way it can protect the other; thus Black can play Re6-f6, and now if White captures he would be trading his queen for a rook. It would be okay if White were using a bishop, but since he only has a queen for the purpose the skewer doesn't work out.