The principle
Everyone has a weakness, a hole in their armour. That weakness is usually an insecurity, an uncontrollable emotion, a need, a small secret pleasure. Find it and use it.
The application
The raw version is manipulation. The civil version is that knowing what someone is sensitive about is what makes you not step on it. Negotiation, leadership, and friendship all require this literacy.
The defensive tell — how to spot it being run on you
When a colleague reliably finds the one phrasing that gets under your skin, the targeting is not coincidence. Acknowledge the trigger to yourself — not to them. The defence is internal recognition; the worst move is to escalate.