What is good posture? Typically one thinks about sitting up straight, and it’s a little like that with your fingers as well.
Beginner players, especially if they are short, can develop the habit of “sweeping” their fingers across the keys. Their arms are too low to allow the tips of their fingers alone to touch the keys. What happens is that the hand lays down because the elbow pulls it down. The fingers are then resting flat on the piano key instead of the tip.
The trouble with this is that it reduces efficiency. When the “sweeping” begins to happen, the fingers are doing a lot more work than they need to. The extra motion causes fatigue even faster and prevents the necessary fluidity needed for more advanced songs. Scales are difficult, finger positioning becomes difficult, chording, arpeggios and accidentals can become a challenge as well. These kinds of things are all dependent on strong, correctly postured fingers.
So, to state it directly, here’s the tip: Keep your fingers curved, and fingertips on the keys.
As I type, I realize that the posture should look like good computer posture. You don’t type with your fingers straight down, nor do you type with them flat (where your knuckles are almost touching the keys). No, your arms are fairly parallel with the ground and your fingers have a slight bend. Your fingertips are the only things hitting the letters. That’s the point.
Of the benefits, another music teacher and performer named T.H. Gillespie who has seen this problem often agrees:
Curved fingers not only facilitate rapid action for fast passages, but ensure greater touch control.
As much as possible, your fingertips should be the only thing that hit the piano keys.