Limb IK gives you additional control over your legs, which was previously only possible with your arms. Characters can now kick, dance, jump, sit, and do other things, while developers have control over things like knee bend direction and ground layer.
Forward Kinematics is used in a typical skeleton rig. Each bone in an FK rig mimics the behaviour of the parent bone. In the legs of skeletons, IK Rigs are especially useful. The simplest way to ensure that the feet stay rooted on the ground while the body above them is free to move is to use an IK set-up.
Inverse kinematics (IK) is an animation technique that reverses the chain manipulation direction. Rather than working from the tree's root, it starts with the leaves.... The IK solution rotates the upper and lower arms by moving the pivot point of the wrist, known as an end effector, toward the objective.
See Glossary, then click the Layer's cog settings icon and check the IK Pass checkbox in the menu that appears.