On one extreme, they end up being too distracted, and bouncing from one thing to another, never completing a task. ("too hot")
But they also, as a reaction, tend to go to the other extreme. They end up in hyperfocus/OCD, where it's difficult to dislodge them from their current activity. Now, instead of not being able to complete ANY task, now they will only complete THIS ONE task, and will resolutely avoid course-corrections, even when a course-correction is necessary to attain the desired goal. ("too cold")
The goal for an ADD person is to find the "just right" zone, and avoid getting stuck in either extreme. Not changing tasks on a whim, but also considering task changes if it's truly warranted.