If you zoom in close to some of the RED Tour pics you’ll see that her plunging necklines, like on the red bodysuit from “The Lucky One” (and others), that it’s not actually a plunging neckline. There’s a nude, sheer panel of fabric called an “illusion panel”. It’s to give the appearance of a particularly risqué neckline, but none of the accidental revealing while you’re doing fast-paced movement (like on tour).
You can see it best in this photo:
High-res version here.
Taylor Swift Style 