Practicing relaxed and what is Sung

Don't do internal martial arts completely relaxed. That is slumping and will put your body out of alignment. This will lead to injury. You need to do it in sung state, which means using just enough muscle and tendon power to keep body in a REQUIRED position. This implies that ALL your muscles, big and small, external and internal are strong enough, so that if engaged together, they only need to be engaged up to the sung level. If you are physically week, then sung level of muscle activation is probably way above your normal strength and tension level, and does not feel like being relaxed. But you need to achieve this muscle tone level if you want to be stable.

Here are Erle Montaigue lessons on sung.

Part 1
Part 2