Can FK and IK works simultaneously?

I downloaded a rigged model from BlenderKit and noticed that FK and IK can be used simultaneously,it is useful !!!but how is this achieved? here is file


man.blend (4.5 MB)

When you have a simple chain of bones, Blender has an ‘Auto IK’ option that will do that. It’s in the ‘Tool’ tab of the ‘N’ panel in the 3D viewport.

https://docs.blender.org/manual/en/latest/animation/armatures/posing/tool_settings.html

It’s not as fully featured as a proper IK setup. You won’t get foot-roll mechanics or anything fancy. But it does the job for some quick posing.