Anal tugging/gripping. A tight butthole clamping down and not letting go.

This is a quick and dirty model I trained up as a test with trainflow.

no trigger required. the degree of pulling can be quite absurd, especially with certain artists, so feel free to experiment with lower strength for a more subtle effect.

Make sure to give hybskgks28275's version a try as well:

https://civarchive.com/models/2583107/anal-grip-for-anima?modelVersionId=3041926