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Undermining (also known as removal of the guard, or removing the defender) is a chess tactic in which a defensive piece is captured, leaving one of the opponent's pieces undefended or under-defended. The opponent has the unpalatable choice of recapturing or saving the undefended piece. A possible response is to sacrifice the piece whose defense is being undermined before capturing the piece which just took the defender.
Kramnik-Topalov in the seventh round of the 2004 Linares chess tournament reached the diagrammed position with White to play. The black knight on a4 is defended only by the black pawn on b5. White undermined the knight with 1. Bxb5. The game continued 1... Rxb5 2. Rxa4, with a net material gain of a pawn for White.
The obvious continuation is 1...Nxb2 disposing of the unguarded knight. If 2.Kxb2 Rxb5+. This regains the pawn for Black. However, Black avoided 1...Nxb2?? because of the strong rejoinder: 2.Rb3 when White is winning a piece.