Some people can do it without having to "learn" how, but some people can't.
(Consciously expand the ribcage, that is.)
Imagine you are stuck wearing a nylon band around your chest, a couple inches below nipple level. It's bothering you, so you want to get it off. But you can't use your hands, so you have to just pop it off by expanding your ribs sideways.
Can you do that?
If not, here's another thing to try, courtesy of Jeanne Deva, who wrote The Contemporary Vocalist:
Stand, feet hip-width apart, good relaxed posture, arms hanging down at your sides, palms facing forward. Let your mouth be slightly open, so that air can come in and out of your mouth. Without making an effort to take a breath in, slowly raise your hands up sideways away from you, arms straight over your head. (As if you were going to make a snow angel.) You should feel air enter through your mouth if you did this move correctly. This means you expanded your ribcate.
Then, let your arms come slowly down. You should feel air come out of your mouth as your ribcage relaxes out of its expansion.
OK, next step: Do the same thing, raising your arms overhead and feeling air come in through your mouth as your ribcage naturally expands. But this time, when you bring your arms back down, keep your ribcage lifted and expanded.
If you do it right, this time, you will NOT expel air from your mouth, because you are maintaining the ribcage expansion.
Make sure that your shoulders and neck aren't tight when you try this. It's important to be able to expand the ribcage by only using intracostal (between-the-ribs) muscles, not your entire upper torso.
OK, got that now?
Now, just try for the ribcage expansion without lifting your arms overhead. Hopefully you have the feeling of it now.
If you aren't used to doing this, keep practicing, first with the assistance of your arms going overhead. You may simply not be used to using your muscles this way, and they will get stronger and more flexible as you practice!
(c) 2011 Adrienne Osborn