Why are China’s rocket launch countdowns so different from America’s countdown passwords?

If you watch the live broadcast or video of the Long March 7 launch, you will find that it takes longer for the flames to erupt than in the previous Long March series. Simply put, China's rocket countdown is the ignition countdown. After ignition, it takes a certain amount of time for the engine to reach full thrust before it can take off. There are contacts on the launch platform. If the rocket moves, it will be connected. The precise take-off time will be immediately measured and entered into the measurement and control system. If you watch the live broadcast of China's rocket launch, after the "ignition" and "takeoff" commands, someone will soon report, "Takeoff, XX hours, XX minutes, XX seconds, XXX milliseconds." American rockets all use a countdown to takeoff, or the time when the main engine thrust is fully established as mission zero. The difference is that none of China's current launch vehicles use a containment release device. Whether the rocket takes off or not depends entirely on the thrust of the engine. Once the thrust reaches the rated value, it will naturally take off. Since engine starting is a transient process, the transmission time of the command will not be exactly the same, and the actions of each engine will not be completely synchronized. The take-off time is the result of this process and cannot be controlled manually, so we can only count down to ignition and then measure the take-off. time. All mainstream launch vehicles in the United States use a containment release device. That is, the rocket is "locked" on the launch pad before the moment of takeoff. The engine is started a few seconds before the scheduled takeoff time. The containment release device will measure the thrust of the engine. If it reaches The rated thrust will unlock the rocket and the countdown to zero is the time to fly the rocket. Since the containment release device allows each engine to operate in the stationary state of the rocket for a short period of time, the impact of asynchronous thrust between different engines can be eliminated, and its action time can also be controlled more accurately.