Hi Sarah,

It's a combination of velcro sheets and slotted tabs. A picture being worth 1000 words, it's best if you watch the video. When the link for the Altra 75 page opens up, go to the last video on the bottom right and you'll see everything you need to know. You can adjust it in, out, up, down, until you get the fit just right.

http://www.arcteryx.com/Product.aspx?EN/Mens/Packs/Altra-75