How do I use Foot IK?

I checked Foot IK in my animation for a humanoid character but it's not doing anything. There's no options for me other than checked or unchecked, so I have no idea about what I'm supposed to do to get it to work. Having no options is confusing, how does Unity know what layers to raycast down to? Thanks in advance.

 

There is only one layer, and I have IK turned on for that layer



My unity version

 

Bump. Is anyone able to describe exactly what is this checkbox for?

In my experience, I use "FootIK" feature to correct "feet sliding" problem.
Example:
1. Import one humonoid IDLE animation into Unity
2. It could happen that, even if this IDLE animation in Maya has feet perfectly fixed on the ground, in unity there is some sliding (even if I bake into pose XZ root transform position). Can someone tell me why is this happening?
3. To solve this problem, I turn on FootIK.