The arm made with a brass rod is the rotating lever. Front side of the lever is connected with the key lever via link. As a key lever depressed the lever rotate around the stem and press the string against the friction wheel. The vibration energy of the string is transmitted to the soundboard via the bridge. Corresponding to the key depth, the string moves up and down, rubbing the side wall of the saw tooth shaped portion of the bridge. This provides dynamic expressions. Precisely speaking the saw tooth shape of the bridge is a partial circle with a radius equal to the arm of the rotating lever.