HSPE - Honey Select Pose Editor
This mod can control IK better that the original game. You can see how to install in the above link.
After you installed HSPE，In studio IK mode on ; Push H to activate HSPE interface
In this tutorial, we would use HSPE advance mode. Just tick it. Another black window interface would be shown.
For beheading, we needed to move her head. Push "FACE". We switch to "Position".
Edit XYZ asix then it's done. You don't needed to use some objects to hide something / put her body on the ground and push her head under the ground anymore.
But for the cross-section... You still needed something to cover that... :D
Some emotions would remain tongue. In the Character Tree, cf_J_Tang_S00 is contorlling tongue. You can set its Position XYZ asix too.
The tutorial is finished. I would show a demo at below.
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(R-18 / NSFW)