This is a rigid-link kinematic simulator: every segment is a non-deforming rigid body connected to its neighbor by a rotational joint with a fixed limit, rotated directly to the angle you set. It is a robotics and biomechanics teaching model, not an exact replica of a biological hand.
Degrees of freedom (DOF) are the independent ways a joint can rotate. A simple hinge (like a fingertip joint) has one DOF; a saddle joint (like the thumb base) has two. This model exposes 26 rotational axes across the forearm, wrist, palm, thumb, and four fingers.
Anatomically coupled mode mimics real tendon-driven coupling: the fingertip (DIP) joint automatically follows the middle (PIP) joint, and the ring/little finger metacarpal bases automatically cup as those fingers curl — just as they do in a real hand, where a single tendon drives both joints in sequence. Independent mode exposes every axis as a free input, useful for robotics exploration where each joint might be driven by its own actuator.
Palm cupping refers to the transverse arch of the hand: the ring and little finger metacarpal bases rotate and shift slightly forward so the palm curves around an object during grasping, rather than staying flat like a rigid board.
Visible axes vs. independent inputs: all 26 axes are always visible and animated in the 3D scene. The number of independent inputs you can directly command changes with the mode — fewer in coupled mode (some joints are derived), the full count in independent mode.
Not modeled (future extensions): soft-tissue and skin deformation, tendon/muscle dynamics, tactile sensing, collision and friction, contact pressure distribution, and physical grasp forces. This tool visualizes joint kinematics only.