"Nothing, really," Ianto says, shrugging. "I mean, they can technically see us here. We're not invisible. But they just aren't perceiving us here. Once you step off the block, they they are free to pay attention to you as they see fit, but while you're standing on here, you're kind of just like a shadow in the corner of a person's eye. That's why it's called a perception filter. I just think Invisible Lift sounds better, is all. We've got another one just like this on Myfanwy so we can let her out to fly around. She'd be a right terror, if we kept her cooped up in the Hub all the time."