Inverse kinematics, used extensively in character
animation.
IK is the opposite of FK, forward kinematics. In FK, you move and rotate bones in order to make movement. For an arm (eg), you rotate the shoulder, the arm, the forearm, then the hand, to maybe get your character to
hold something.
In IK, you'd start with the hand using an IK target, and move up from there if
necessary.