Inverse Kinematics. When it's on (for the legs), you can move the feet and the legs will follow but the hip will stay still, or you can move the hip and the feet will stay still. You can't bend the legs directly at the thighs or shins when it's on. For myself it's a marginally useful feature. There is IK for the arms as well, but that defaults to being turned off. I have found that IK sometimes causes problems with conforming clothing. Usually turning it off fixes things, unless the clothes are also set up with IK. As I said earlier, if both the garment and the figure have IK, it's important that both are on or both are off.