The positive reinforcement training method is a practical result of science, called behaviourism. Behaviourism is a branch of ethology (science of animal behaviour) that was developed in the course of the 20th century mainly in the USA and that, in a lot of experiments, investigated and verified general principles of how different behaviour patterns emerge or cease to exist, depending on whether they were or were not beneficial to the animal.

Behaviourism actually relies on a simple and logical principle: the behaviours that were successful for the animal will be reinforced with further repetitions, while the behaviours that were not successful will be suppressed. A successful result of a behaviour can be seen either in a situation when the animal has got rid of something unpleasant (that is negative reinforcement), or has got something pleasant (positive reinforcement). All that leads to logical conclusion that if we train by positive reinforcement, we support selected behaviours by having the animal get something pleasant in connection with them.
It could be said, in simplified words, that positive reinforcement training equals to training for rewards. But that would be oversimplified.  First, you often won’t get along with mere CUE - BEHAVIOUR - REWARD dialogue (i.e. I give a command to the animal, the animal performs a behaviour and I will give it a reward). Imagine a seal that has just jumped through a ring above the water and should get a fish for it. Before reaching the fish, the seal will do at least four other behaviours (it has jumped, dived, surfaced, run to get the reward), and so it is by far not certain whether it will consider the fish a reward for having jumped through the ring, or for having run to the trainer. It is highly probable that next time, it will not jump through the ring but stay next to the trainer in order to get another fish. In short, rewards usually arrive late, and in order to tell the animal YOU HAVE PERFORMED SUCCESSFUL BEHAVIOUR JUST NOW, a signal called "bridge" is inserted before the reward. In case of seals, it can be a whistle: when the seal jumps through the ring, the trainer blows the whistle to confirm to the seal that "it was right just now, so come to get the reward".

So the dialogue occurring between the trainer and the animal in positive reinforcement consists of the CUE (given by the human) – BEHAVIOUR (performed by the animal)  - BRIDGE (given by the human) – REWARD (given by the human, eaten by the animal). Bridge is not the only "strange gadget" in positive training. The trainers' vocabulary includes dozens or even hundreds of words like variable reward schedule, primary and secondary reinforcer, contrafreeloading, behaviour momentum, timeout, extinction burst……in short, it is not very simple. A peculiarity of the positive reinforcement methodology consists in the fact that the animal trained by this method lather performs the learned behaviour for anybody who is able to repeat correctly the learned cue. It is similar to the situation of the sportsman heading towards his goal - such sportsman can be happy with advice of any trainer, and the animal also follows its goal, and therefore takes each chance offered, if it understands it. Because the cue, or command, is really a chance and not an obligation in positive training. And this is perhaps the most beautiful point in this methodology. And this is also why the full dialogue with the animal in positive reinforcement training consists of these words:  I WANT THE CUE (animal) - CUE (human) - BEHAVIOUR (animal) - BRIDGE (human) - REWARD (given by human, consumed by animal)

The positive reinforcement training was transferred from laboratories to practical life particularly in the second half of the 20th century. With the help of a sophisticated methodology supported by scientific foundations, performances of big sea mammals in which the human hardly could use force or pressure started working. The method, beneficial in many respects, spread also to zoos, dog handlers, parrot or cat breeders, to some degree also among horses, and today it is even used in company management and employee motivation. Modern animal trainers are today able to train seals and dolphins in open sea without any restriction, to teach the animals to undergo voluntary blood taking, to hold the body out for syringe or even to have a probe inserted into the stomach, standing voluntarily without motion during the whole time. There are plenty of possibilities, and we certainly have exhausted all of them.