The graph compares displacement and planing hulls and the effort required to keep them going.
For displacement hulls, you can see that coasting provides for a lower cadence while maintaining speed.
Try this: on a flatwater day, while paddling normally, simply stop paddling, and you’ll see the “V” bow wave continue to occur for quite a while, indicating that you’re still moving through the water.
For planing hulls, thrust must be constant to keep the hull planing, else speed falls off rapidly, and additional effort is required to get the hull planing again.
When a displacement hull is pushed beyond its hull speed, it begins to exhibit the same characteristics as a planing hull. Think about it: you’re effectively trying to raise the bow of your kayak/canoe out of the water to get it go faster. You can do that, but the effort required is significantly higher; you get less bang for your buck. In a sprint, this might be worth the effort, but on a longer paddle, it’s doubtful you can maintain that profile.