The Tahitian stroke is constantly lifting the nose of the canoe or, in this case, a stand up board, and it’s designed to build momentum. Your hands, torso and shoulders return to position automatically, on a circular, continuous path, and only travel a short distance. To build speed you increase the cadence, and it’s a lot easier to do that because the stroke recovery is shorter and starts earlier.
You need to get a lot of power in that first bit of pull, and to do that you need your shoulders and torso engaged. If you are doing the catch right, then your shoulders and torso are already in the right position for a stroke. So as soon as the paddle is in the water you twist your torso and shoulders back straight. Again, to get the feel for this I want you to overcompensate and do this as a sudden jerk. You can mellow out later and get things smooth, but we want all that energy firing on the first foot of pull, and then it ends.