One of the great challenges of Pilates is relaxing the muscles that don't need to be working. Undue tensions around the body can interfere with your ability to work the muscles you are trying to strengthen. The set of muscles that most people have a hard time relaxing is their face. It seems that majority of people, my clients anyway, think they can do a roll up (an exercise close to a full sit up) by just tensing their jaw. The same holds true for one-legged bicep curls on a BOSU, which is a serious challenge, but clenching your jaw is in no way going to help you access your core. In fact, tensing your face, be it your jaw, brow, or forehead, activates the muscles in your neck and shoulders, and this unwanted tension can lead to injuries. So next time you are working out, check your face. If it is tense, relax it. I find deep sighs or "motor boat mouth" help get that job done.