Rigging: Digital Orthopedia - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 260

Put the 3D cursor in the center of the
first knuckle (where the finger joins the
hand) of the index finger. Add a bone
and place its tail at the second knuckle.
Extrude twice and adjust to finish the
finger arc, like in Figure 9.22 . Notice
how the arc of the bones is actually a
little more acute than the arc of the
finger mesh. We do this on purpose.
When you straighten your hand and
fingers fully, the fingers actually curve a
little bit past a flat plane. An IK chain
can only be stretched until it straightens
perfectly, meaning that it would be dif-
ficult to use IK on a system that needs
to flex past straight. If the finger mesh's
arc is less pronounced than the bone
chain, though, the resulting deformation
at the flat full IK extension will actually
be a little backwards on the mesh.

Figure 9.21 The scaling curl construction.

With the 3D cursor in the same place,
use the Add command to create a new
bone there. Select the end point of the
finger arc and use Shift-S to snap the
cursor there. Select the tail of the new
bone and snap it to the cursor. With that
end still selected, extrude the small bone
that will be our IK target. In Pose mode,
select this new end bone, then Shift
select the last bone in the finger arc. Add
an IK constraint to the finger arc by
pressing Shift-I. Remember to set Chain
Length to 3 (the number of bones in the
finger) in the Bone Constraint properties
so the IK doesn't affect the entire body.
Now, scaling and rotating the large bone
in Pose mode causes the finger to curl.

Figure 9.22 The finger arc.

Before we get too happy and start duplicating the finger, there is some housekeeping to do. Name all of
the bones descriptively. Disable Deform for the control bone and its IK target end. On the Inverse
Kinematics panel for each of the finger arc bones, disable all but one of the axes. Finger joints only bend