My technician said that he can ‘tap the pins’ to increase the holding capability of the tuning pins. What does that mean?
Your technician was probably referring to his driving the pins a little deeper into the pinblock (see earlier diagram as to location of pinblock) to increase their ‘holding’ capability. By driving the tuning pins deeper into the pinblock, it gives the block more pin to hold on to. Ultimately, the piano will stay in tune longer which was probably the reason your technician was suggesting the procedure in the first place. In the photo, driving the pins deeper (pin tapping) is not a simple procedure in a grand piano as the pinblock has to be supported before any tapping takes place. This avoids possible damage happening to the pinblock laminates (separation) and thus permanently damaging the pinblock. If that happens, then the entire pinblock would have to be replaced. Supporting the block is not necessary when tapping tuning pins in upright pianos.
Driving the pins is usually viewed as a stop-gap measure that postpones repinning and restringing the piano. The main reason for having to tap the pins is because the pinblock has dried out over the years and thus has lost its ability to hold the pins securely enough for a tuning to last. 