In both Freeform and Footstep animation, pivots allow you to rotate the biped's hands and feet around various points. The biped's hands and feet have the same number of pivots and pivot location is similar. By activating a pivot on the ball of a foot you can rotate the foot around the ball of the foot for example. Pivots are only active if the biped hand or foot is in world or object coordinate space. In a walking motion you can pivot on the heel first, then the ball of the foot and finally the toes. Pivots are essentially extensions of the IK chain.
By setting a planted key for the hand it is anchored in world space, you can move the biped or the collar bone and the hand remains planted. Pivots on the hands makes it easy to animate hands and fingers.
To understand Biped's interactive IK limb manipulation, it is useful to distinguish two types of limb joints: primary and secondary.
The primary joints are the shoulder, elbow, hip, and knees. Animals use primary joints to coordinate the positioning of hands and feet since these joints have the most influence and flexibility in positioning tasks. Even the placement of fingers and toes over specific spots is the task of the primary joints.
The secondary joints are the wrists, ankles, toes, and fingers. These joints are typically used for grasping and support rather than positioning, so they most frequently exercise independent joint angle control suited to a particular task, such as rotating the foot to raise the heels during walking or the curling of the fingers around an object. Because these joints have little influence on end effector position, they are rarely engaged for positioning tasks. For example, if you want to place your finger on your nose, you will most naturally rotate only your shoulder and elbow, keeping your wrist stationary.
Biped mimics the IK behavior of "natural" systems. The primary joints are used for IK positioning while the secondary joints are independently and precisely controlled by the animator. The secondary joints will not rotate unless the animator explicitly selects and rotates them. Therefore, if an IK pivot is set, and the limb is interactively manipulated, (say, the finger is moved onto the nose), only the primary joints adjust to satisfy the IK constraint. The secondary joints remain in place and will never "drift" in a IK solution, they simply obey the values set by the animator. Because there is no drift for the secondary joints, the animator does not have to tediously assign "tension" values to hold them in place.
If an IK pivot point constraint has been set, a byproduct of Biped's "natural IK" is that:
1) Interactively rotating a secondary joint will always adjust the primary joints to hold the IK pivot in place.
2) Biped's "special rotations", the forearm/calf X axis rotations, rotate the elbow about the axis from the shoulder to the wrist and rotate the knee about the axis from the hip to the ankle. These rotations give the animator a direct way to interactively rotate the primary joints while holding the IK pivot in place.
3) Interactively translating a limb part to move the IK pivot will only alter the primary joints.
4) Interactively rotating a primary joint will move the IK pivot with it.
After an IK pivot has been placed, applying 1) and 2) gives the animator exact control over all possible IK solutions. The IK pivots can be easily moved by applying 3) and 4). In general, these "rules" don't need to be remembered since the system works in an intuitive way. The animator merely needs to position the pivots as desired, set the IK constraint, and then fine tune the posture of the limb, if desired, by rotating the secondary joints and using the special rotation to adjust the primary joints.
Note: During animation playback, the "Ankle Tension" parameter is used to set the relative importance of the interpolated ankle joint over the interpolated knee joint for intervals in between keyframes. This is only relevant to Biped's interpolation.
Using IK constraints and pivots on the biped hands and feet allows you to animate quadrupeds.
IK Key Info Rollout
To use pivots
Select a biped hand or foot that's in world or object space.
Turn on Select Pivots in the IK Key Info dialog.
Select a pivot in the viewports.
Turn off Select Pivots.
Rotate the hand or foot around the selected pivot.
Set two consecutive keys with the same pivot if you find that you are not rotating around the selected pivot.