When you draw a chain, you usually draw it with a bend to be able to predict its behavior when using IK. This bend is called the chain's preferred angle, as described in Preferred Angles. Softimage tries to stay as close as possible to the preferred angle when IK is invoked.
You may need to adjust a joint's preferred angle to get the correct skeleton structure for the animation that you are trying to create. When you change this angle, the IK is solved in a new way, affecting the movement of the whole chain.
To change the preferred angles
Select a chain bone and open its Kinematic Joint property editor.
Enter values for the bone's Preferred Rotation Angles X/Y/Z axes.
Keep the Kinematic Joint property editor open so that you can see the Preferred Rotation Angle values, then interactively rotate a bone in a 3D view using the Rotate tool in the Transform panel. This way, you can see how the changes you make affect the way the chain reaches its goal and the preferred angle rotation values that are set.
You can still rotate the bone and see where the effector will be placed, such as for keying the position of the effector.
You can animate the preferred angles of a joint by setting keys for the Preferred Rotation Angles settings.
When the animation is played back, the keyed values replace the preferred angles of the joint before the chain's IK is resolved. This means you can change the behavior of a chain as the animation progresses.
You can select the Force IK option in the Kinematic Chain property editor to trigger IK every time you modify the preferred angles (see Forcing the Effector's Position). This way, you can immediately see the effect on the chain.
If you use an expression or linked parameter to control the preferred angle, its values will be overridden if you are using the SI|3D solver (see Choosing How an IK Chain Reaches Its Goal).
Skeleton