To me, it seems like person B magically knows when person A is about to stop talking and takes up the conversation the second A stops, no pause in between.
From my own observation of conversations, it looks to me like a cooperative thing. Usually, A doesn’t simply stop talking, having said her piece, after which B takes a turn. The handoff from A to B (or to C, or to D) is an interactive process involving both voice and body language extending for a perceptible interval (below one second) around the moment when one stopped and the other started. B does not know when A is going to stop. Neither does A. Instead, B has something to add and sees a juncture at which a handoff can happen. There are many places in a conversation where it could occur, but doesn’t, or did, but might not have done.
I’d draw an analogy with improvisational dance or music, if I had experience of those—maybe someone who has could comment. Or perhaps friendly sparring in martial arts. Or sex.
ETA: Which is why it can be a mistake to wait until A has “finished”. Depending on A’s conversational style and the subject matter, “finishing” may simply be the wrong concept to apply. In a one to one conversation it may seem to you like you can’t get a word in edgeways, while A is wondering why you aren’t saying anything and she’s having to do all the work.
From my own observation of conversations, it looks to me like a cooperative thing. Usually, A doesn’t simply stop talking, having said her piece, after which B takes a turn. The handoff from A to B (or to C, or to D) is an interactive process involving both voice and body language extending for a perceptible interval (below one second) around the moment when one stopped and the other started. B does not know when A is going to stop. Neither does A. Instead, B has something to add and sees a juncture at which a handoff can happen. There are many places in a conversation where it could occur, but doesn’t, or did, but might not have done.
I’d draw an analogy with improvisational dance or music, if I had experience of those—maybe someone who has could comment. Or perhaps friendly sparring in martial arts. Or sex.
ETA: Which is why it can be a mistake to wait until A has “finished”. Depending on A’s conversational style and the subject matter, “finishing” may simply be the wrong concept to apply. In a one to one conversation it may seem to you like you can’t get a word in edgeways, while A is wondering why you aren’t saying anything and she’s having to do all the work.