Abstract: This invention can have industrial application in every swimming pool with water propulsion system (e.g. with jets, propeller, paddle-wheels, etc.) like swim-spas, where the user has to swim against a water current. It offers a technical solution to the problem that occurs at conventional pools of this type, where the water flow remains at a stable strength, until someone interferes manually. The new mechanism reacts to the swimming of the user automatically, since it recognizes the acceleration or deceleration of the swimmer via optical digital sensors and sends the necessary control command to the water propulsion system, raising or lowering the intensity of the water flow accordingly and so putting the swimmer automatically at his initial position. The advantage of this invention is mainly, that it gives the possibility to the serious swimmer to exercise in a technical right way and to the trainer to gain access to useful data.