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Your body speaks before you do, so teach it to be on your side - Dale Carnegie
The core message is that your non-verbal communication, your posture, your face, your gestures, sends a powerful signal long before you even utter a word.
Your body language tells your confidence level before Meaning Factcheck Usage

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Your body language tells your confidence level before you speak - Leil Lowndes
Your posture, your eye contact, the way you hold your hands, they all broadcast a message about your self-assurance, or lack thereof, long before your voice is ever heard.