...speak the truth with heart and tongue. RB 4:28
Benedict is suggesting that we allow the truth of the heart to come to the surface and make it accessible to others. The work of asceticism is designed to produce over time a transparency by which our inner truth becomes perceptible. The truth that we ARE and SPEAK is meant to bring the glory of God more visible to everyone we meet.
A quote I found from Marianne Williamson gives food for thought about this line in the Rule of Benedict.
Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure. It is our light, not our darkness that most frightens us. We ask ourselves, who am I to be brilliant, gorgeous, talented, fabulous? Actually, who are you not to be? You are a creation of God. Your playing small does not serve the world. There is nothing enlightened about shrinking so that other people won't feel insecure around you. We are all meant to shine. We are born to make manifest the glory of God that is within us. It's not just in some of us; it's in everyone. And as we let our own light shine, we unconsciously give others permission to do the same. As we are liberated from our own fear, our presence automatically liberates others. (A Return to Love - Harper Collins, 1992, p. 190)