Wing Chun's Elbow Force - POWER from the Shoulder girdle

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Here is another well known term to all Wing Chun practitioners: Elbow Force!
What does it actually mean? some think that it just means to 'sink' your elbows down and keep the shoulders sunk. Is that it? But how can we do that? If we use muscles to 'hold' the shoulder down won't that lead to lack of mobility?! Wouldn't that be very restrictive for mobility, power and speed? So if not holding the shoulder or elbow in specific position, what is Elbow Force?
What is your idea of Elbow Force?