In therapeutic music, we consider both technical and spiritual sides, as both greatly influence the result.
If the string on a guitar is plucked very hard, it sounds bright and fun, but the vibration may exceed the pressure limit which produces elastic waves, resulting in fewer transverse waves being generated and less hypothesized effectiveness in sound therapy. Alternately, if the string is plucked softly, it moves gently and creates transverse waves which we believe will reach the human cell. Ideally, a performer is aware of how he/she can 'effectively vibrate' an instrument to optimally produce more transverse waves.
Phrasing is also an important consideration. There needs to be enough spaces between notes so that the instrument is able to vibrate and produce transverse waves before the next note is played.
As with spiritual healers in many cultures, a sound therapist prays while performing, believing that the power of healing is derived from Almighty God and not from his or herself. If a performer tries to "show off" and plays an instrument hard, more longitudinal waves are generated than transverse waves (as mentioned in "Sound Generation") which may result in excitement, but not relaxation. It's important that a performer casts aside their ego, intentions, and whatever else is carried while performing.
Being HUMBLE is the most important factor when performing therapeutic music.