You need to redirect a character from striking at symptoms to hunting the decision-maker; or a rule for revenge/thriller plots: the tool can't be punished, only the will behind it.

Jisbella's instruction is the novel's turning point in method: Foyle's brute force is useless against a trap because steel doesn't think. The target is never the instrument, it's the mind that directed it. This sentence converts Foyle from a revenge animal into a strategist, and it sets the template for every later scene where Foyle climbs the chain of responsibility instead of smashing the surface.