Pay attention to what special words your partner uses and use them in conversation with them.
“You’re speaking my language,” is a true statement. See if you can add tone to this, again matching them first and sometimes you can lead with tone and volume.
If you prefer to experiment, break rapport. See how far apart you have to be before you feel emotionally out of sync.
Who notices the break first?
Pay attention to who usually breaks rapport first. Pay attention to how it is broken, because there is likely a pattern and you can then pay attention to how to easily establish rapport and connection once more.