What it costs them
Then there's the other person, and the jump makes it hard to see them, because the jump is the moment you stopped looking.
The unfinished sentence stays unfinished. Marvel's redirected patients rarely completed what they'd started. I'd expect the same of anyone. The thing they were going to say doesn't get parked and picked up later. It gets dropped, and if it mattered, it comes back in a worse form, sharp and out of context, or it never comes back at all, and they carry it.
The lost thought. Itzchakov's finding was that being listened to well gives a speaker clarity about their own view. The reverse is that being cut off takes it away. The person who was three words from working out what they thought is left not knowing, and may not get another run at it.
Feeling overrun. Not insulted, exactly. Something quieter. The sense of having started to say something and found the space already occupied. Youngquist's listeners called it dominance. The person on the receiving end mostly calls it nothing, and just says "anyway".
The withdrawal. People don't fight a jump. They shorten. Sentences get front-loaded so the point is out before you arrive. Stories get skipped because they'd need a run-up. Over time the person becomes, in your company, less than they are, and you can't see it because they're always like that with you.
The child. Children raised by an interrupter learn two things. To talk fast, and that what they were going to say is less important than what the adult already knows. The second lesson is the expensive one. A child who's learned it stops bringing the tail, and the tail is where the thing they're worried about lives.
The colleague who prepares less. Priya spent two days on the north region figures and got to present half of them. Next quarter she'll spend one day, because the second day's work never gets heard. The jump made her worse at her job, in the specific way that matters to you.
And the friend on the phone. They rang to tell you about their mother, and what they got was your mother. They'll ring less. When they do, they'll keep it light, because light doesn't need a tail.
The partner who says "fine". I described them in chapter 12, and I'll add only that "fine" is what a person says when they've learned that the full answer won't be reached. It isn't sulking and it isn't a lack of interest in you. It's an accurate forecast of what happens to sentences at your table, and the forecast is what changes first when the sentences start getting through.
I said in the preface that the book doesn't ask you to talk less. It doesn't. All of this is the cost of one thing, the start of your reply landing before the finish, and the whole of it can be recovered by moving that start about a second.