Psychologists Tanya Chartrand and John Bargh documented what they called the 'chameleon effect' in 1999: people unconsciously mimic the posture, gestures, and speech patterns of people they're interacting with, and — critically — the study found that being mimicked (without realizing it) made people rate the interaction as smoother and the other person as more likable, even though they couldn't identify why. Mirroring appears to be a byproduct of empathy and rapport running in the normal direction: we naturally start to mirror people we're already connecting well with.
This is exploited deliberately in sales and negotiation training as a technique — consciously matching someone's posture or pace to manufacture rapport — and it does work to some degree, but forced mirroring is also detectable when it's too obvious or delayed, which reads as mimicry rather than genuine connection. The more durable version of this skill isn't performing mirroring as a trick; it's paying close enough attention to someone that natural, honest engagement produces the same unconscious synchrony the research describes — which is also the version that doesn't collapse the moment someone notices what you're doing.