To try it, put your fingers on a flat surface with your palm lifted up, then bend in your middle finger. Then try lifting your thumb, little finger and index finger one by one. Done? Now try your ring finger. HAHAHA. Sorry.
Your index and little fingers each have their own extensor muscles (the ones you need to be able to move them), but your middle and ring finger share one. So when you lock your middle finger underneath your hand, you can't move your ring finger.