The principle
Anger and emotion are strategically counter-productive in yourself. Provoke them in your opponents and you gain the upper hand; rattled people make mistakes.
The application
A person off-balance reveals more, decides worse, and binds themselves to positions they will later regret. The discipline is to remain calm while the other side is being stirred — and to recognise when someone is doing this to you.
The defensive tell — how to spot it being run on you
When someone keeps pressing on something obviously designed to provoke you, do not engage. Acknowledge once, neutrally, and return to the substance. Their goal is your emotion. Denying it ends the play.