Roller coasters work because of momentum—the quantity of motion from the downhill allows the car to make it up the next rise. Without momentum, the car would merely stop. But few things in the world of ideas follow the same rules.
Ideas have no mass, they don't coast.
Authors fall into this trap over and over again. They believe that a big launch, the huge push upfront, the bending of the media in their favor (at any cost) is the way to ensure that weeks two and three and eleven will continue to show solid growth.
I'm betting that an analysis of the Billboard charts over the last fifty years would confirm that the speed a song makes it to the top has no correlation with how long it stays at the top.
The launch is the launch. What happens after the launch, though, isn't the result of momentum. It's the result of a different kind of showing up, of word of mouth, of the book (or whatever tool you're using to cause change) being part of something else, something bigger.
Fast starts are never as important as a cultural hook, consistently showing up and committing to a process.