This is a rigging practice done in Maya.

Rigging for limbs and spline are implemented both in IK and FK. As shown in the graph, the blue skeleton is the IK control skeleton and the pink one is the FK.

IK/FK Switch

To smoothly switch between the IK/FK control, a control handle is used. By switching the value of the handle to 0 or 1, animators can easily use either FK/IK control. That is, 0 for FK and 1 for IK.

When the switch is set to a certain control method, the other control rig will not be shown.

Feet Control