As you start marker training your dog, you are molding him into an active dog. Your dog starts to problem solve and engage with its handler by trying to do things that will result in a reward whether it's a treat, toy reward, or time spent with you.
Active dogs try to intiate engagement. They try to do things that result in their handler giving them a reward. This isn't an innate ability. This engagement is something that is taught by the handler. Many trainers think they have the wrong dog but these people are wrong. They simply need to learn how to provide an environment that makes the dog want to engage with the handler.
Dogs who don't know engagement are reactive dogs. They wait to be shown what it is the handler wants. They are afraid to try new things and are afraid of being corrected for any mistakes that are made.
Your dog is always teaching you something. You might not know what it is but our dogs are constantly watching and evaluating us. An active dog is trying to figure out how to interact with you in a way that'll benefit him. A reactive dog is simply trying his best to avoid corrections. However, marker training helps add a motivating method that will help produce an active dog.
An active dog will focus on the dog and offer behaviors that may cause his owner to start engaging with him. An active dog will start bouncing around, giving eye contact, laying down, sitting up, or even going to the heel position without being asked. The dog will continue trying things so that he can receive a reward. On the other hand, the reactive dog goes to the training field and waits for the owner to ask him to do something. He simply reacts to what the handler asks and doesn't try to do anything for fear of a correction.
Reactive dogs haven't learned engagement. They have only learned that a wrong move will warrant a correction so they quit trying. They back off and take the safe approach with the thinking, "Wait until he tells me what he wants because I don't need another correction."