Moving a hand to position the arm is an example of inverse kinematics. Biped's inverse kinematics has three parameters that you can vary during the limb's motion by setting them at each key of the arm and leg keyframe tracks. As the limb moves through each key:
IK Blend sets the motion interpolation to be a blend of forward and inverse kinematics. This will allow you to blend swinging motions with hand/foot directed motions. Using an arm to move a hand is an example of forward kinematics. Using the hand to move the arm is an example of inverse kinematics. The default is 0.0, or full forward kinematics.
Body/Object determines the reference coordinate space of the IK path. This will allow you to move the IK path with your character's body or temporarily attach the hands/feet to follow other objects or be attached to world space. The default is Body.
Join to Previous IK Key determines if the key should be part of the previous key (same reference position as previous key).