Confidence is a key component of partner dance. It unifies intent and thereby the body in the communication of self definition and thus all the communication that is based off of that. With confidence dancers will move with intent and unity in their body that communicates proprioceptively to their partner.
Without confidence get problems like paralysis, motions that are not fully integrated with the body creating proprioceptive feedback.
Those who lack confidence often do not have the right amount of tone, leave tension in places throughout the body and thus mess up mechanics.