I don’t have an assembled view, but perhaps it will help to understand how it works. With the twist grip in "FAST" position, speed is controlled by the slider lever under the tank. Now, if you are running in that mode, and want to slow down, turn the twist grip toward the "SLOW" position. That closes the carburetor shaft without retarding the main slider lever. In that mode, it continues to run but slower and somewhat roughly. To smooth it out, you have to retard the spark (main lever).

In other words. the twist grip only works when main lever is in the high speed position.