If you play the violin eventually someone will bring up the idea of playing from the base joints of the left hand. Curve your finger so the fingertip is floating over the string. Then you lift and drop the finger by moving from the knuckle joint, where the finger joins the hand.
This is easy to say, but people sometimes find it difficult to do, which makes perfect sense: Apparently the fingertip is the densest site of nerve endings in the body. You have really acute sensation at the point where the fingertip hits the strings. Back in your knuckle joint? Not so much!
Once you realize what is going on, though, you can put your attention fully on that sleepy knuckle. Hover the curved finger over the string and then enjoy activating the movement from the base joint of the finger.