Unlike your other fingers there’s more tendon sharing and muscle motor unit mixing between the ring and middle finger digitations of the profundus than the superficialis. That is why when you bend your middle finger sufficiently and hold it in place (can be done with one hand) the ring finger cannot move. It’s also known as the “paralyzed finger” effect. The tendon for your ring finger is connected to your middle finger. With the middle tendon stretched (bent) to its length, the ring finger tendon being tethered to it will not be able to lift upwards.
Hope this helps explain it ~~~