How does this work

This is sort-of correct.

Assuming the container makes a perfect seal, if you hold the container under water at the drain, the water will empty out until the atmospheric pressure pushing up from the drain balances our with the atmospheric pressure pushing down on the water in the tub/container. Hence why at the beginning the container wasn’t moving.

When in motion, the container hits the tub and bounces a little (conservation of momentum, some of which is lost to the tub), which produces a negative pressure inside the container that pulls more water in until the pressure difference is resolved, at which point the container gets pulled back towards the drain, thus allows the cycle to continue.

Because a bit of momentum is lost every time the container hits the tub, the amount it bounces decreases until motion stops, which returns to a balanced pressure state. Which is why they had to restart the process in the video to fully drain the tub.

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