First, keep your thumb, index finger, and pinky stretched out and fold under your third and fourth fingers under as shown in the picture below.

Now try to lift up each stretched finger. Easy right?

Now untuck the fourth finger (keeping only the middle finger tucked in).

Try to lift up each finger again. What did you find?

You can’t lift up the fourth finger! It really feels as if your ring finger is stuck on the table.

It has to do with the bones and tendons in your wrist and hand. The tendons in your fingers are independent from one another except for the ones in your middle and ring finger. When you fold under the middle finger alone, it tenses the interosseous ligament connected to the capitate bone in the wrist. This tension causes the movement of adjacent ligaments connecting the ring finger to be restricted.

This has been know for many many years and in the 19th century doctors would surgically sever tendons in the hands of pianists to increase ring-finger independence and allow better playing.