The involuntary contractions, what muscles are they?

So I’ve been using the aneros, kinda just getting started, and also have been listening to the free mindgasm audio pieces.

I’ve definitely felt the shivering/contractions that get mentioned, but it’s always very much my buttocks that are kinda jerking on their own and contracting on their own. Is that what I should be going for, or is it more your internal muscles/PC muscles that should be contacting? Because it doesn’t feel like my buttocks muscles are going to be moving the aneros much.

If anyone could be super detailed about which muscles I would be trying to fatigue I’d really appreciate