Aggression is usually a substitute for a move people don't know exists. When a point isn't landing, the instinct is to say it louder or more times, on the theory that force gets it through. It rarely works, because the resistance isn't a volume problem. It's a listening problem. The other person is bracing to defend, not absorbing what you say.
The move that actually shifts people is quieter: repeat back what they just said, as fairly as you can, before you respond. "So the timeline works if the vendor contract's signed this week, otherwise it doesn't." You're not agreeing, you're proving you heard them in their own terms.
This lowers their guard in a way aggression cannot. Most people in an argument spend half their effort making sure they get to finish their point, because they don't trust they've been heard. Once you demonstrate you heard them accurately, that defensive effort has nowhere to go, and they become available to consider what you say next.
It also exposes weak positions faster than arguing does. When you mirror a position back cleanly and it sounds weak even in the speaker's own words, they hear it too. You don't have to attack it. Aggression makes people defend harder, because now their pride is attached. Mirroring lets a weak position collapse on its own, without you raising your voice.
The IBC course teaches mirroring as the low-force move: repeat the other person's point back accurately before responding, which lowers their defenses and makes them available to actually hear you, no aggression required.