Sacrifice

Rxd5

White's rook captures the knight on d5. The only black piece covering that square, the pawn on c6, recaptures immediately. White has deliberately traded a rook for a knight, a net loss under the fullest exchange the square allows.

Rxd5

The nearest counter-case depends on whether anything answers the move profitably. If nothing recaptures, or if every recapture nets the capturing side less than the piece it would win, the same move is a normal capture or a check. No material is given up, and this attribute does not apply.

The only piece able to recapture on d5, the black pawn on c6, is removed from the board. Everything else, including White's move, is unchanged. Rxd5 now wins the knight, and nothing is left to answer it. The move is a free capture, not a voluntary loss.