Do you mean the Bernoulli bit? In which case, the fast flow of water entering the lock near the top gate causes a reduction in pressure (Bernoulli). The lower water pressure allows the air pressure above to push the water down a bit more than it would for stationary water. As the flow moves back it is “absorbed” by the volume of the lock and so its velocity decreases, with corresponding reduction in the Bernoulli effect. The net result being to create a downward slope on the water surface, and the boat slides down that slope. Perhaps less obvious if you are single handing and not on the boat when top paddles are opened, but on the boat it is very obvious that first, the boat pitches nose down, and then the boat starts to move forward (down the slope). Bernoulli of course just being conservation of energy.
