(Standard and Expert modes) The Orientation Constraints links the rotation of a layer to the orientations of other layers.
This is a very useful tool when you need a layer to stay always aligned with another layer, without translating with it.
You can also use this constraint to link the orientation of a controller to the orientation of any layer in the rig, this is a quick and easy way to control rotations in a rig, without parenting.
Select a layer and click on the "Orientation Constraint" button. An effect is added on the layer to let you select the constraint and set its weight to adjust the influence.
To constrain a layer to several other layers, you can duplicate the effect, and adjust the weight of each constraint.