When he finished the apple he again backed up to the tree and repeated the process. This horse had learned to operate on his environment in order to get something he wanted. He repeated the act that resulted in the reward reinforcement, in this case the apple. When your horse acts on his environment to stop or remove an adverse condition. The removal of these aversive stimuli serves to reinforce your horses' actions.
Most people use the removal reinforcement method when training their horse.
For example, they squeeze their legs to get their horse to move forward, then let up on the pressure when the horse responds correctly. The removal of the pressure is reinforcing to the horse. The On Target Training system focuses on reward reinforcement. When used alone or used together with your training method, your results can improve dramatically. With removal reinforcement training, the only thing in it for the horse is to avoid an aversive stimulus, something he doesn't like.