If a client can't effectively "brace" their abs, it's not a good thing. Most people catch on to this after a while, however, there may be a few clients here and there who have difficulty with this.
Here's an exercise "tweak" that have helped one of my clients gain consciousness of the "bracing" sensation, thus making every exercise safer and more productive. Have client press knees and palms against ball. This facilitates a contraction that reflexively stabilizes/braces the torso, along with producing a posterior pelvic tilt! Ta-da!