Desensitization and counter-conditioning.
Desensitization is a treatment or process that diminishes emotional responsiveness to a negative, aversive or positive stimulus after repeated exposure to it (Wikipedia.org). In the case of dogs, we use desensitization at their pace. To do this we would expose them to a trigger that is causing the stress and wait until the dog relaxes. Once the dog is relaxed at a safe distance then we can move in slightly closer and repeat the method until the distance is closed and the dog no longer gets emotionally aroused by the negative stimuli. It is very important to be fully aware of dog body language as a dog that is too stressed in a situation will not benefit from participating in desensitization. You need to be at a distance the dog finds comfortable, to begin with, and slowly move that trigger closer, once your dog is relaxed at a distance they choose. What you are hopefully achieving by using this method is turning a negative response to stimuli into a positive response to that stimuli.
Counter conditioning is the functional analytic principle that is part of behaviour analysis and involves the conditioning of an unwanted behaviour or response to a stimulus into a wanted behaviour or response by the association of positive actions with the stimulus (wikipeadia.org). When we use counter conditioning with dogs we are pairing something amazing with and refusable with a situation or stimuli they have previously felt fearful about. This process is simple but can be time-consuming when carrying out counter conditioning you are making new neural pathways to associate a past negative response to a positive one and because of this again it's taken at the dog's pace and rushing this process will not achieve the desired response. It could go the opposite way. To carry out counter conditioning you would reward at the sight or the sound of the negative trigger but keep it low at a distance the dog finds comfortable to start with and slowly increase the pressure.
Using both of these methods together can help a variety of reactive behaviours. Using an example of a dog that is showing reactive behaviours towards bicycles. I would have one person with the bike, away at a distance the dog is relaxed. Once we know the dog isn’t showing any signs of stress or anxiety. Waiting until the dog calmly notices the bike then reward and keep rewarding with every glimpse towards the bike. Once this is achieved I would take one or two steps towards the bike, remembering to always keep checking the dog for signs of stress, if all is well then repeat the treatment process. Repeat the steps each time coming gradually closer until you get up close to the bike, this is then a party with the dog! Making a new memory of fun things happening at the sight and closeness of the bike. The more fun and rewarding you make this process the more the dog will be willing to work with you. Building new neural pathways take time so these exercises may be done over a few days or a few weeks. Counter conditioning and desensitization go hand in hand and used correctly can achieve amazing results for reactive fearful dogs. I think a key thing here to mention is no one likes to be stressed so when the dog realises that there is nothing to fear and starts to relax they will happily work with you. Sensi