To my knowledge, nobody knows. There was one description of the launch sequence that implied that the hold-down clamps were released at engine ignition and the ship took off as soon as thrust was greater than its weight (which, as I understand it, is how Soyuz works), but I have never heard confirmation of this from any other source. It seems an odd way of doing business, since you’d usually want to be able to confirm you have all engines at full thrust before committing to flight. Falcon 9, for example, has had several cutoffs after engine ignition when engine monitoring saw something it didn’t like.
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