By stopping moving forward when the puppy pulls on the leash, what is the handler demonstrating?

The handler stopping movement when the puppy pulls on the leash is demonstrating negative reinforcement. In this scenario, the handler is removing the consequence of forward movement (which the puppy desires) when the unwanted behavior (pulling) occurs. As a result, the puppy learns that pulling does not lead to the desired outcome of advancing on the walk; instead, it leads to stopping.

When the handler ceases to move forward, the puppy may eventually learn that walking calmly on the leash is the behavior that allows forward movement to resume. This reinforces the desired behavior without the use of physical correction or punishment. The removal of the potential for forward movement serves to encourage the puppy to walk without pulling, illustrating the mechanics of negative reinforcement effectively.