A piano string presses tightly against several friction points along its length. When it is tightened or loosened during tuning, this friction prevents the new tension from being evenly distributed throughout the string.
The tuner pounds the note to bounce the string up off the friction points slightly to allow the tension to equalize. If this didn't happen, the tension would equalize on its own when you sat down and played loudly, and the piano would slip out of tune.
The tuner pounds the note to bounce the string up off the friction points slightly to allow the tension to equalize. If this didn't happen, the tension would equalize on its own when you sat down and played loudly, and the piano would slip out of tune.
