Friction is the mechanism that holds tuning pins. It is not an easily found place where the pin will stay. The pins have very fine threads- you can barely feel them with your fingernail. They are driven into laminated hardwood. Holes in the hardwood are .015” or so smaller than the diameter of the tuning pin. The 150 lb avg tension on the string- attached to the tuning pin- cause the pin to “unwind” a tiny bit as you bring the string up to pitch and let go of the tuning lever. The microscopic hardwood fibers actually grab the threads of the tuning pin and each pin can be grabbed by the fibers at different points.
Fortunately, most newer pianos have improved pinblock materials with more laminations that hold the tuning pins more securely. Even so, you have to go back and retune most pins as you tune the piano.
Experienced tuners have learned the knack of how far to bring the pin above pitch to have it come back down perfectly in tune when you relax the hammer and hit the key with a strong blow to make sure it stays.
To make things a little harder, there are 4-5 different tension points on each string. For example, the distance between the pin and the pressure bar, between the pressure bar and v-bar, between the v-bar and the 1st bridge pin (sounding length), between the 1st and 2nd bridge pin, and then between the bridge and the hitch pin. When the pitch is changed during tuning the tension may be different in these different sections of the same string. A strong blow as you tune each string is required to equalize the tension, or it will equalize itself as it is played- causing it to go out of tune even if you set the tuning pin in the right place in the pinblock. This is the primary cause of much frustration as you are learning to tune. Basically, it’s impossible. Vic Smith, piano store owner and my first employer, told me it would take at least 1000 tuning to become “comfortable” in the tuning process. Turns out, for me, it was a lot more than that.
Some tuners tell customers that pianos need to be tuned every 6 months or it will hurt the piano. The truth is that they are leaving a word off. "...will hurt the piano tuner" is what they should say. Pianos can go 50 years or more without "hurting the piano". I know because I work on several of those a year. They just need more tunings to get them to stay up to pitch. Every 6 months is actually a good rule of thumb for tuning frequency. Every three months is better. Some churches do them every month. It mostly has to do with the tuning tolerance of the user. Most people don't have a clue when their piano is out of tune. Pianos typically drop in pitch every year if they are not tuned