Of the many tools available to police forces, the police dog (of the "K-9 unit") is one of them. Trainers take advantage of the dogs' capacity for learning and obedience to adapt it to police tactics. Police dogs, typically German Shepherds, can be trained to pursue and attack suspects; seek out drugs, bombs and other illicit materials; and perform other functions.
Positive and Negative Reinforcement
Police dogs are trained via a combination of positive and punishment. Put simply, positive reinforcement provides rewards for good behaviour, while punishment induces some unpleasant consequence for bad behaviour. Police dogs are given treats and verbal praise for successfully completing a task or following directions. Conversely, their trainer may issue a mildly painful punishment (such as a sharp tug of the leash) or a harsh command to express disapproval. The combination conditions the dog to perform the proper actions.
Dog trainers can use a number of tools, but two are most common. An electric collar worn around the dog's neck delivers a mild shock via remote control to discourage certain behaviours. Trainers must be properly trained in shock technique before using it. Additionally, trainers often use a type of bungee cord (the variable bungee) to teach the dog bite restraint techniques. The dog is tied to the cord and when it pulls against the cord's tension, the cord snaps back, pulling the dog with it. This technique is used to discourage the dog from re-biting when releasing a suspect's arm.
Bite and Hold
The bite-and-hold technique is one of the basic K-9 manoeuvres. A fleeing suspect is pursued by the dog, who bites firmly onto the suspect's arm and does not release until instructed by his handler (who typically arrives on the scene after the much speedier dog.) Bite-and-hold is not designed to inflict injury (although that can result) but rather is designed as a method of physical restraint. Generally, police will only use this tactic against a suspect who is a physical threat.
Dogs used for the purpose of detecting drugs undergo a different kind of training. Handlers constitute training exercises as a sort of "game" for the dog, in which real-life drug detection is simulated. The dog is rewarded for successfully locating drugs. In training, as well as in real-life application, handlers sometimes place drugs for the dog to discover in order to keep its Pavlovian mechanism fresh and active and ensure the dog doesn't get "bored" and lose interest (and therefore ability) in the activity.