When your tail or tails fully engulfs your own body or another's body.
As seen here, https://danbooru.donmai.us/posts/6538140 here, https://danbooru.donmai.us/posts/978618 and here, https://danbooru.donmai.us/posts/979628
Tail_Wrap just wraps a tail around someone, and Tail_Rape mixes this idea with using tails like dildos when this doesn't inherently have to be sexual.