Another way to understand what I can't do is to lightly take both of your hands and place
the fingertips on the surface of
a table in front of
you. Now, bend your second joint
on your fingers while holding the first joint (the one closest to the fingertips) flat. I have no problem doing this with my right hand, but on my left hand, unless I push down on the first joint of my middle, ring, or pinky fingers, they remain curved. My index finger works OK but is weak.