An abusive sometimes angry critique of the victim's actions or work. Such a painful and embarrassing evaluation that it feels like the authority is trying to tear a new asshole for you. (Assholes being an embarrassing body area.)

Secondarily, tear a new asshole may refer to total domination by an opponent similar to a gay rape where the attacker didn't even bother to find to the actual asshole.

When someone goes to the gym and spends 90% of the time on their phones scrolling through social media

To display a level of awesomeness that eliminates all competition. To make someone, usually an audience, feel as if they have been torn a new anus by your amazing display of skill.

What one might do with a drill-shaped penis.

To be extremely annoying to someone; to mistreat somebody to an extreme.