I need to talk a bit about the contradictory opinions about how to use your abs when doing an exercise that is not ab specific, like crunches.
Some fitness trainers promote abdominal bracing to work your core and protect your back when doing things like lifting weights. Bracing is explained how you would hold your abs if you were "bracing" for a punch to your gut. Now as a Pilates instructor, there is no way I can sound impartial about this and I don’t like “bracing.” Here's why: In a worse case scenario it makes people push their abs out (not good for stabilizing the spine), as well as bear down on their pelvic floor (not good for your core), and at best it simply works the abs isometrically (doesn’t change their length) and does nothing to correct faulty posture.
I think it is much more beneficial and more supportive to work the abs by pulling the abs toward the spine in a kind of in and up fashion. This also helps to engage the pelvic floor, which believe it or not is part of the core. Often engaging the deep abs (aka the transverse abdominis or TVA) helps to correctly faulty posture and alignment as well as create a lift in the torso so essentially the torso is supporting itself decreasing the pressure on the joints in your lower body.
So you want to find your deep abs? Check out: The Skinny On: The Deep Abs.