Actually, the rocking is due to the need for the captain to use speed and power to turn, but lose the forward momentum almost as quickly. Basically, he is "rocking the boat" by going quickly into helm-counterhelm-helm-counterhelm, etc. in cycles, which creates drag and thus quick loss of forward speed with power on and causes the rocking motion. To effect the turn, you just hold on one cycle (helm or counterhelm) more than the other a little bit to get the turning effect.
It's text book shiphandling. I've had to do the same with Gate Vessels parking with 90 degrees off-jetty wind at Esquimalt's A3 jetty, with the steamers sticking their god-damn sterns 40 feet off the end of A2, blocking part of the approach.