It happens quite often that I see organists play with their fingers extended. They can't seem to hold the rounded palm.
That's because they depress the keys with a wrong place of the finger.
You see, the right way to touch the keys is with the tip of the finger, just below the nail. Except for the thumb which plays with the side of the finger to the outer side of the nail.
So when you extend the fingers, the last joint sort of breaks making the shape of the finger not rounded but flat.
This is good for wind instrument players, by the way. But for keyboardists, we need to make sure of the correct position of the palm.
Here's the key: imagine you have a tennis ball or an apple in your hand. All fingers are held together and make up a cup.
That's the correct position.
PS And don't forget to clip your fingernails.