Video is added just so we all have a reference footage, and can all play this back at 0.25x if required.

But basically they're saying that by analysing your movement, Chisato can tell if you're going to change from your original fist or not.
She's probably reading something on your final upstroke, as both players are supposed to commit on (at the beginning of) the downstroke.

If you have your hand behind your back and simply decide to whip it out - Chisato has to choose between 3 options (paper, scissors rock), not just 2 (counter fist, counter not-fist).