With the goal being that the dog needs to remain in drive and keep focused throughout the exercise before it gets it's toy or food. If it breaks focus, the exercise is finished and there is no toy or food for the dog. You must start all over and the next time, back the reward up a few steps. We force him to think about what he must do. This stage of training teaches a dog control over himself.
Some dogs go so high in drive that they stop thinking.
An example of this is when the learning routine is changed just enough to create a mistake. In heeling this can be a speed change in the walk. It can be a right turn or left turn. Some form of compulsion is almost always used before an exercise has been fully trained. Compulsion takes drive out of the dog.