Qawl bi-l-mūjab accepts something in the adversary's formulation without conceding the conclusion that the adversary meant to establish. The responder retains a portion of the expression or its admissible implication while redirecting its force.
First identify the initial assertion and the conclusion its speaker hopes to draw. Then identify precisely what the response accepts and how the accepted expression is made to support a different implication.
A bare denial or unrelated counterargument is insufficient. The technical interest lies in turning the opponent's own admissible words or implication against the intended result.