Constraints are a way to control an object’s location, rotation or scale. Constraints can be used on objects or bones.Constraints are mainly used in animation. It can also be used on objects for controlling in a static sense. While constraints are used on the object as a whole it can also be used to control a vertex group or a bone part.
To apply a constraint to an object(or bone), first select it.
Constraints can be activated from the Properties panel by clicking on the button having two chain links as its image icon. Hovering the cursor over this icon will display its name.
When you have an object in the 3D window, constraints can be activated by going to the header bar which is below the 3D windows, clicking on Object ->Constraints-> "Add constraints with target". Keyboard shortcut for this is Shift+Ctrl+C. This will open the constraints menu panel. Here click on the particular constraint. This will open the particular constraint's menu window where required setting should be made to activate the constraint.
More than one constraint can be applied to an object which is known as cascading constraints. All the constraints from top to be one below till the last one will have its effect on the object.
A constraint when applied to an object may be temporarily deactivated by clicking on the eye icon in the constraint window, to closing it.
Further the influence setting which has a value varying from 0 to 1 can be animated. Setting the influence value to zero will also deactivate constraint.
