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Do they go back to ensure everyones foot is on the block to a precise mm, or to ensure that the runner didn't flinch or move their foot from the block 1/1000 of a second early before the sound hit their ears, taking into account the distance from each runner's ear to the closest speaker playing the starting sound?

The extreme level of precision, I suspect, is not the same at the finish and the start.

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They do, in fact, have humans who check to make sure that everyone starts behind the starting line. And both humans and computers which check to make sure that nobody starts early. Two notes on "starts early." One - start is any visible bodily movement, not the crossing of the start-line. So any flinch, whether in the direction of the start-line or not, is a false start. Two - if you start inside of 0.1 seconds of the start-gun, it is declared a false start, since the assumption is that no human can react faster than that. The precision at the start is quite high!

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