There are many answers to this question. One is that there can be a change in one encounter. Pianists often come to me in the midst of a busy concert season. Unless there’s a severe injury, just putting into place a few changes can make a profound difference.
The following few items are examples of some of these common changes:
– correcting the chair height
– explaining how the finger, hand and forearm must be connected
– showing the correct wrist level
– showing the correct height of the forearm when playing
– making them feel the freedom of the forearm (a missing link for most players)
– feeling the support and freedom of the forearm in putting down a key
– feeling the support of the forearm in helping to move from finger to finger without stretching.
This is the short answer. The long answer is that there are many complex skills involved in playing the piano. As in any other discipline, it takes time to learn, deepen and make a habit of those practices, which can improve the playing in ways that are surprising and unexpected.