[fazool]

When an elastic member vibrates, it temporarily elongates. That elongation changes the frequency of its vibration.

So, *WHILE* vibrating at a pitch, the string changes its pitch.

The best example is tuning your guitar. When you strike a string very hard the string actually stretches a little and increases the tension and raises the pitch.

You can see this yourself with a tuner. The harder you strike the string the more sharp it goes.

The same with tall frets: as you press down harder you are stretching the strings more and making them go sharp.

As the sound board distorts it will induce similar effects.

So, I imagine that a stiffer soundboard reduces some of that effect.