Bird dog training at its core is behavior modification reliant upon catching your dog in the thought process and being able to read their emotions. Doing so allows us to predict behaviors before they occur, thereby maximizing the potential for learning. Also, the character of the dog at that moment and the behaviors they are exhibiting dictates the amount of pressure required to gain compliance.
Pressure comes in various forms. Understanding pressure and how to use it is entirely another article in itself. Basically, pressure can be generated in a variety of ways. Food pressure is when the dog feels internal pressure to perform for treats. The leash and collar is an example of physical pressure. Standing in the proximity of your dog places spatial pressure on them and using corrective tones places verbal pressure on them. Social pressure can even come from the competition of using other dogs.