The term Concentration Kick refers to any kick that is performed slowly, to develop strength and muscle-memory. The term is mostly commonly used in Tang Soo Do, though the exercise is often used in taekwondo as well. Side Kicks and Back Hook Kicks are often practiced in the "concentration" style.
To perform a Concentration Kick, you normally find something to hold onto or lean against, such as a bar or wall. You usually keep the base leg in its correct position throughout the kick. For example, if you were doing Side Kicks in a concentration style, the base foot would be pointed toward the wall. Then the kicking leg should move slowly through the kicking motion, often pausing at full extension in order to build strength. Concentration Kicks are repeated in sets, as a form of exercise.