To repeat a worthy refrain: two enemy rooks on the same diagonal should cause you to think about skewers. That is what White did, and as a result he just played Bf4, bringing us to this position. Black is threatened with the loss of the exchange. He can put a pawn in the way at e5, but since the pawn would be loose this is a good example of a useless interposition. White just plays Bxe5 and the skewer is renewed. What Black needs is a way to seize the initiative, breaking a piece out of the skewer with a threat that White has to take time to fend off. The natural choice is a check if one is available, so Black plays Rd1+. After White moves his king, Black has time to move his other rook out of the White bishop’s path as well.

Lesson: examine every check, even when you are under attack. And examine any checks the enemy can play in the midst of whatever you are planning. This position also shows why “relative” skewers—i.e., those not involving the enemy king—are, like relative pins, easier to escape than the absolute variety. The pieces involved have greater freedom of movement, and so can create bigger threats to save themselves.