In turn, [the tuning pins] are embedded in a wooden block under pressure called the [pin block], they are squeezed in place. So, every time you hit a key, a mechanism system activates a hammer to strike the string therefore temporarily producing an extra load on the string, consequently giving extra pressure against the friction of the embedded pin.
Under 'normal playing' this extra load doesn't much affect the status of the string or cause any perceptible movement of the pin itself. But the harder you play, the more pressure adds up on to the pin. It eventually gives up some of its position, therefore lowering the tension of the string (e.g. detuning). A common example is young kids who love to play with their fists on the keyboard...
In the case of ‘hard’ playing, the detuning could occur quite rapidly. But the same effect will also happen the more frequently you play on your piano. Therefore, for the dedicated musician who loves practicing even just one hour a day, I would suggest to have his/her piano tuned up once every 6 months.
The only one time when it is necessary to hit the keys hard, is at a certain point, when I tune the piano (or as any piano technician should do). When adjusting the tension of a string, by turning the [tuning pin], I need to find its proper position, by administrating a firm blow to establish an equal strength between friction and tension for the string to remain stable. This is also called 'settling the string' or 'settling the pin'.
Sometimes after a tuning session, in the following days or week, some keys may lose their position and get out of tune again. This usually depends of the condition of the strings before working on it (age, rust or tightness of the pin etc.). In these cases, call me back and I would gladly fix it next time I am in your area.