When the heat comes back
You'll be angry again, possibly this week. The feeling will arrive as quickly as it always has, because nothing in this book changes your first second. What I'd like is for that not to alarm you. The return of the urge doesn't mean the decision has failed. It's the situation the decision was made for.
The urge comes as a bodily state with a push behind it, and it's usually accompanied by a thought. The thought is nearly always some version of "this time is different." This time they really have gone too far. This time a calm word won't do. This time it's justified.
Notice that the thought can be correct. They may well have gone too far, and your anger may be entirely fair. The rule was never "stay calm unless it's justified," since it always feels justified. The rule is that a justified point will still be justified in twenty minutes and will be better put. So you don't have to win the argument with yourself about whether your anger is fair. Grant it, and leave the room.
It's best not to debate the urge at all. You'll be debating with a mind that has already taken a side, and it argues well. Use your exit line and move your feet. The physical act of walking to another room does more than any amount of reasoning in your head.
An urge behaves in a predictable way when it's not acted on. It climbs, it peaks, and then it falls. While it's climbing it seems as though it will go on climbing until something gives. It won't. If you can watch it with a bit of curiosity, noticing the heat in your chest and the speeches writing themselves, you'll find the peak comes sooner than you expected. Every time you sit through one, you add to your own evidence that the urge is something you experience and needn't obey. I'd put more faith in that evidence than in anything written here.
Some specific difficulties will come up.
You'll catch it late. At first you may only notice when you're already shouting. In that case stop in the middle of the sentence. It feels absurd, like braking in the middle of a junction, and it's entirely allowed. "I'm shouting. I'm going to stop and take my twenty minutes." A row abandoned halfway through is a real success, and the people around you will see it as one. With practice, you'll catch the signs earlier.
The other person follows you. Some people can't bear an unfinished argument and will come after you down the hall, still talking. This is why the exit needs to be agreed beforehand, in a calm hour. If it happens anyway, repeat the line once, with the time you'll be back, and keep going. Leave the house and walk round the block if you need to. Don't explain further, because explaining turns into arguing about the exit.
The other person is the one shouting. You don't have to match their volume, and you don't have to stand there and take it. "I want to hear this, but not like this. I'll be back in twenty minutes" works in both directions. If someone's anger makes you afraid for your safety, that's a different situation, and leaving and getting help come before any technique.
You'll feel strange, even fake. If you've shouted for twenty years, speaking calmly while angry can feel false, as though you're acting. You aren't hiding the anger. You can state it outright. What you're dropping is one way of showing it, and the new way will feel odd for a while. Any new habit does at first.
You may also feel flat. The outburst gave you a surge, and its absence can feel like something's missing, particularly if rows were the main way strong feeling got expressed in your house. That usually fades. I'd rather warn you than have you take it as a sign that you've lost something important.
One caution about keeping count. It's tempting to tally days without an outburst. I'd advise against making much of the streak, because a streak makes one bad evening look like a disaster, as though the count has gone back to nothing. It hasn't, and chapter 13 explains why.