Removing the defender - Undermining

You capture a piece which defends an opponnent piece i.e. you remove the defender.
So this piece became undefended .
deflection example 1
diagram 1

Black Knight defends the Rook . Bishop captures it , Black King captures the Bishop. 1. Bxe7 Kxe7 then see: diagram 2

deflection example 2
Diagram 2

Now Black Rook is undefended and White Rook captures the opponent Rook. 2. Rxf5

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