Disagree with that, you'd ground through the feet, but the movement would be initiated from the hara/koshi
Ok... thing is it's all connected.
It really all starts in the cerebral cortex with the intent to move, travels down the neurons (is it the pyramidal tracts?) to the biggest muscles first, and travels outwards to the smaller muscles. Any more detail than that and I'm out of my depth. core musculature acts to transmit the work done by the big muscles in the lower body to the limbs. Essentially, it's all connected...
However - the fellow moving the boat in the movie is doing real cormorant boat rowing, with a real boat. I wonder what would happen if we actually learned how to do the movement with a boat, rather than some imaginary (or real) person holding our wrists, and then moving to people. that's if you want realistic training...