When you train karate you exercise your mind to focus on what you are doing by eliminating other thoughts from your mind. The training of Karate demands your whole presence of you at the same time, it means, to keep your body and your mind in the same place at the same moment.
Katas are not easy to learn and execute, they demand all our attention and focus because every single move has a technique designed very carefully by the masters through centuries of accumulated experience. Kata is a fight against an imaginary enemy and you can't visualize him unless you are fully concentrated and this happens only after repeating the exercises many times. With the time you gain the ability to focus and concentrate deeply by eliminating all unnecessary thoughts.
Attention Deficit Disorder
Many people with attention deficit disorder find in the training of Karate a help to cure it. After few months of training they find themselves with the ability to concentrate in one subject for longer periods. The reason is that Karate-do is a way to get into your own self joining your soul and your body in one only being. In Karate-do you learn to empty your mind of all unnecessary thoughts that block your concentration.