The system takes the current FK rotations of a limb and then fits them to the IK target. This flexible approach includes support for IK chains that rotate on any axis, not just one as with other IK systems. It is also easy to build in custom components, such as constraints, to customize how the system works.
The fact that the IK solution is derived from the FK solution means that in Setup mode it is important to keep the FK and IK configurations as close as possible. A good trick is to click the Match FK To IK button before you start to edit the limb in IK. For complex limbs, such as a spider’s, it is recommended that you position the limb in FK, turning on IK only when you are finished.
With complex limbs, rotate the bones in FK so that they sit on the footplatform.
Because you can rotate the limb bones on any axis, you can have bowlegged IK chains for gunslinging cowboys.
The difference between IK and FK in CAT is minor. The way a limb behaves when manipulated is much the same whether in IK or FK. The difference is that in IK the child end of the limb, for example the palm, always tries to follow the IK target. In FK the limb doesn't have this restriction.
Use IK when a limb must have a fixed end position. The most common example of this is animating legs walking, where feet must be planted on the ground without sliding around as you rotate the leg bones. Other examples would be arms holding onto other objects, such as guns or steering wheels.
You might be surprised to find that rotating the IK target does not rotate the limb. While in the first instance this may seem strange, it actually makes CAT's IK system more flexible and isn't a problem.
One simple way to deal with this is simply to rotate the upper leg bone separately (see Tweaking the Limb Bone Rotations While Animating, below). Another alternative is to constrain the upper leg rotations to the Footplatform.