Start by resting your hand on a table before you reach the keys. Drop your arm, allowing your fingertips to fall into position as if they were being set down naturally. You will see that the fingers adopt a gentle curve. This is much more useful as a working piano hand shape than a rigid claw. At the piano, curved fingers should serve to facilitate movement, not lock every joint.
The primary reason teachers insist on curved fingers is that flat fingers make control of the keys difficult. Collapsing fingers often force a player to use arm movement to depress each key. The tone can become uneven and directional changes can feel awkward. Rounded fingers give each key a specific point of contact, enabling the fingertip to press down without the whole hand plunging into the keyboard.
Tension arises when players attempt to “keep” that curved shape too persistently. The knuckles raise, the wrist becomes stationary, and fingers float as though afraid to touch the keys. All of this can make even five-finger pattern exercises feel fatiguing. Rather than “keeping” the shape, think “returning to the shape.” The hand can move, release, and return. Piano playing is not static; it is a series of small movements which require release.
As an exercise, try playing C, D, E, F, G with the right hand in five finger position. Before playing each note, check that the wrist isn’t raised and that the thumb isn’t tucked tightly into the palm. Allow the fingers to drop each note, and release after each note. Repeat this with the left hand in a nearby position. If a finger is weak (especially finger 4 or 5), do not press harder. Instead, slow it down and make the movement smaller.
Try keeping a tiny bubble under your palm. The hand is curved enough that it isn’t collapsing the bubble, but soft enough that you aren’t gripping the bubble. This is more useful than copying the shape you think a hand “should” have. Every hand is unique, and the aim is controlled comfort. If the wrist, shoulder, or forearm tense up as you focus on the fingers, stop and shake out the hand before continuing.
Curved fingers also link to the sound you produce. Play one short measure and see whether the sound of any notes stands out as louder than the others. Unevenness in the sound can be a sign that a finger is stiff while others are collapsing. Repeat that measure at a slow tempo, not to get louder, but to produce an even touch. A metronome can be useful here, if set slowly enough to allow the hand to release between notes.
The sign of success is not a perfectly-formed hand, but one that can play a pattern, maintain a curved shape most of the time, and still feel loose at the end. After this kind of exercise, lift your hands off the keyboard and see if your fingers bounce up as they fall or if they feel tired and stiff. This small test can tell you whether your curved fingers are helping you or fighting you.