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.
[2] Kramnik–Topalov in the seventh round of the 2004 Linares chess tournament reached the diagrammed position with White to play.
Black may attempt 1...Nxb2?, disposing of the unguarded knight and hoping for 2.Kxb2 Rxb5+ to regain the lost pawn.
After 2.Rb3, however, Black can only save the knight with 2...Rxb5 3.Rxb5, leaving White an exchange up instead of just a pawn.