I think the OP is pretty hilarious - cremation? Satanic? What?
If, during cremation, the corpse sits up and writhes - then it's because the funeral preparers didn't do their job properly. The tendons of a body should be cut before burning. Otherwise, once burning begins, the tendons will basically shorten as they burn up, giving the impression that the body is moving about.
Case in point - when Rasputin's body was burnt, it was described as attempting to get up. Which led to people thinking that Rasputin was some demon in disguise. The reality: they didn't cut his tendons.
Religious people can't be bothered with actual facts such as these... Don't you know that? If it's not in the bible, it's demonic/satanic/whatever...