Astral shifting is when you go to to the astral world and shift. This is possible because in the astral plane, laws of physics don't exist there. You could look like a manticore in the astral if you feel comfortable looking like this.
Mental shifting is when you are physically the same, but you are mentally different, like a cat for example. Sometimes animals are mental shifters. As an example: If a cat is raised by a dog from a young age it is still a cat, but its instincts are like a dog; is still a cat in every way though. This is about real shifting.