Pressing the string with the tip of your fingernail gives a clear sound. After a while, a notch will form in your nail where the string has been. Though a horror for the manicurist, it’s helps prevent the string from slipping out of your nail. That said, as it grows over time, the notch will get deeper as you continue playing. The notch eventually becomes deeper than the thickness of the string. Thus, not only will you lose a clear sound, but the nail will scratch the sao, slowly wearing down the wood. A notch in the nail is encouraged, just make sure it doesn’t get too deep.