So, Foot IK is the thing that makes the game change the position of the legs and feet so that they are always sitting properly if you are standing on uneven terrain. If one foot is on a curb while the other on the ground, it raises the foot on the curb by bending the knee. If one foot is on a surface not flat, it turns the ankle. You have to turn this function off for animations or else this happens because it is trying to make the foot level with the ground underneath it:

I spent all day searching for this. It is very poorly documented, and nobody had posted like “this is a common thing you have to do” - If you are doing bondage animations like I am, or sex animations, it IS a common thing you have to do. So let me save you 20 hours of your life. It is so stupid simple. Some people mention mods that disable Foot IK, but that is dumb because it disables it for everything in the game via the Debug.SetFootIK function. Some people mention scripting to do it, but that is too much work when it only applies to certain animations. I know there is a way to do it within the animation itself too because SexLab is doing it, but I was unable to figure out how to do it in 3DS Max.

The Good And Easy Solution is you add an option to the FNIS declaration of the animation. FNIS has the AV option for AnimVariable and there just so happens to be one for turning this off on an animation-by-animation basis.

o -AVbHumanoidFootIKDisable dcc-dm-chairbound-a1 chairbound1.hkx dcc_dm_AobjChairA1

The option goes after the type value, o in this case for “animation with animated equip”. The - is what starts the option block, if you had multiple options they are separated by comma. Also notice no space between AV and bHumanoidFootIKDisable. This tells FNIS to disable FootIK only on the actor running that animation for the duration of that animation.

Now any time that animation is used, the feet will not be screwed with, and it will magically go back to normal after. Zero Effort. If you add this to your FNIS file you will need to regenerate the behaviour with the blue tool, then rerun the normal orange tool.

There are more AnimVariables you can set too, there is a list of them here on the CreationKit wiki.

[Same post on Tumblr] [What I was working on that required me to figure this out]