Excerpt from this article:
“Since it’s a ring of light, you have this beautiful neutral zone in the middle and a ring of light around the face, so it gives a soft glow along the edges,” explains Musick. “It’s not a single source hitting your face, so it really helps wash out any blemishes. You don’t need to worry about light placement. You just put it right in front of you and it illuminates you appropriately. It’s really easy.” You’ll need a light stand to attach it to and that’s about it. Most come with a mount in the center of the circle for a camera or phone.
But as you can see here in a few (horrible) selfies I took at Racked HQ both in natural light and with hideous overhead office fluorescent lighting, I no longer have undereye bags or weird facial blotches. It also makes a fetching ring in your pupil that really makes your eyes pop, though if you wear glasses this reflection poses problems.