IK and constraints
Pose a chain by dragging its end instead of rotating every joint, and keep bones inside the limits a rig needs.
These controls live on a selected Stretched bone in the Properties panel, under Inverse Kinematics and Constraints (see Bones basics for the Stretched / Static distinction). A Static bone makes a natural IK target: place one at the hand or foot you want to drag and point the chain's Target at it.
Setting a target
Target picks any other bone in the same Bone layer as the goal. Once set, the chain from this bone up toward the root solves toward the target automatically whenever you move it, instead of you rotating each joint by hand. Ignored by IK excludes this particular bone from being adjusted by a solve elsewhere in the chain, so a root you never want disturbed can sit out of it. Turning a bone into a handle stops it being a target, and a handle parented into a chain sits where the solved rig puts it rather than lagging behind it, so the chain no longer whips when one is placed inside it.
Arc IK Solver switches the chain from the default cyclic step-by-step solve to a smooth arc, which tends to look more natural on a chain with just one or two joints.
IK Strength
IK Strength, 0 to 100% and defaulting to 100%, blends between the IK solve and the bone's own forward-kinematic rotation. At 100% the bone follows the target exactly; lower it to let some of the bone's own posed rotation show through, useful for a joint that should mostly follow the goal but keep a bit of hand-animated character.
Stretch limits
Min IK Stretch and Max IK Stretch let a bone lengthen or shorten to reach a target that is otherwise out of range, rather than leaving a visible gap. Min IK Stretch ranges 0 to 100% (default 100%, meaning no shrinking) and Max IK Stretch ranges 100 to 500% (default 100%, meaning no stretching). Raise the maximum to let a limb reach believably further than its rest length.
Angle constraints and independent angle
Angle Constraints, off by default, reveals a min and max angle relative to the bone's rest angle, -720 to 720° and defaulting to -90°/90°. Turn it on to stop a knee or elbow from rotating past a believable limit, whether you are posing it by hand or it is being pulled by IK.
Independent Angle, off by default, makes the bone hold its angle in world space and ignore its parent's rotation, useful for a head or a held prop that should not tip over every time the body it is parented to leans.
Squash and stretch
Squash & Stretch, off by default, thins a bone perpendicular to its length as it stretches and thickens it as it compresses, the classic cartoon volume trick. Amount, 0 to 200% and defaulting to 100%, scales how strong that effect is once it is on.
Control bones
Control Bones let one bone drive another's Angle, Position or Scale as a percentage, Moho style, without any parenting relationship between them. Pick a source bone for each channel (Position gets separate X and Y sources), a Ratio of -500 to 500% for how strongly the source drives it, and a Delay of 0 to 240 frames so the driven bone can lag behind rather than move in lockstep.