In the following positions there is a rook fork waiting to happen: two enemy pieces on the same rank or file, one of them loose, or one of them the king, or both. But there are pieces between them, or between the rook and the forking square, that have to be gotten out of the way for the double attack to work. Again it is important to recognize the basic visual patterns involved—enemy pieces on the same rank or file, suggestive of a fork—without being thrown off by obstructing pieces that you might be able to remove.
Look for visual patterns in the position to the left. We see a familiar layout of Black pieces, with the king and a loose knight—prime targets for a fork—on the seventh rank. If White could get a rook between them, the knight would be his. The Black pawn at e6 is the only thing in the way. The natural method for getting a pawn out of the way is to take something it protects. Here it protects the pawn on d5, so White takes it with Bxd5. If Black replies e6xB, now Re7+ forks king and knight. The net gain is a pawn.