Animate > IK/FK Keys
> Enable IK Solver
This menu item turns the current IK handle’s Solver Enable attribute on or off. The status of the Solver Enable attribute determines whether an IK handle’s solver is active or inactive.
When Solver Enable is on, you can only pose your joint chain using the IK handle. When Solver Enable is off, you can only pose your joint chain by directly manipulating its individual joints (FK).
You turn Enable IK Solver on and off to switch between inverse and forward kinematics when posing a joint chain using both animation methods. See Posing skeletons.