In rocket launch videos, we often observe liquid propellant rocket engines running at full thrust for several seconds before actual liftoff. We understand that liquid propellant rockets consume a significant amount of propellant before liftoff, (between 0.5% and 0.75% for the Space Shuttle).
We also observe that rocket tanks remain connected till the last seconds with quick disconnect lines. See for instance this slow motion images of the Space Shuttle liftoff when the quick disconnect quit contact at minute 9 and minute 29.
Hence my question, is/can the tank be filled up through the quick disconnect line until the last seconds before liftoff, in order to maximize the rocket efficiency?