The alert is probably one of the most important exercises to teach a personal protection, security or police dog, yet is something that is misunderstood and neglected by many trainers. Trainers will typically start the dog by giving the alert command, followed with the decoy agitating, causing the dog to fire up and bark, and the dog is then sent in for a bite. The problem with this sequence of events is that the dog learns that his cue to get aggressive is the agitation from the decoy rather than the command word of the handler to alert. What happens when you need to send your dog on a passive suspect? The dog gets confused and fails to engage because there is no movement! The correct way of training an alert on command is as follows:
The decoy always starts out passive and the handler alerts the dog on the passive subject. Once the dog shows aggression, the decoy reacts to the aggression either by pressing the dog in defense or by fleeing in prey. The dog learns that his aggression makes the decoy move!
If this is done well, you can point your dog at anyone, give an alert command, and expect aggression and focus. This teaches the dog aggression on command, rather than on the context (movement or threat). If you can put the dog in an aggressive mood on command, he will be ready to react immediately and not be caught off guard.
For the full article see Component Training in Controlled Aggression by Jerry Bradshaw