Firstly, the string starts vibrating when initially struck with the bachi. That vibration will influence the following hajiki technique. When you press your finger on the vibrating string, the vibration will make the sound of where-you-pressed louder than if you were to press a stationary string. A vibrating string is great for executing a vibrant uchi (hammer-on), but makes hajiki more difficult. (Again, the only pitch we want to hear is from P.3!)