The "coupling" of finger tendons is mostly between the middle (large) and ring fingers. To demonstrate this place your hand palm down on a table top. You should be able to lift all your fingers independently. Now flex your middle finger underneath so the finger nail is down towards the table and all the other fingers are stretched out resting on the table as before. Chances are it's now impossible for you to raise the ring finger, but you should still be able to raise the first and little fingers.