This menu item sets keys on all the current IK handle’s keyable attributes and all the joints in its IK chain. When you use Set IK/FK Key, Maya performs additional operations to ensure a smooth transition between IK and FK.
You can use this menu item instead of Animate > Set Key when you want to key IK and FK animation on the same joint chain. We recommend that you use Set IK/FK keys when animating a joint chain with both forward and inverse kinematics.
Also, when you key attributes using Set IK/FK Key, the resulting animation curves display as solid or dotted lines. The animation curves appear as solid lines when the animation is fully driven by IK, and as dotted lines when the animation is fully driven by FK. For more information, see Graph Editor overview in the Keyframe Animation chapter of the Animation guide.