The Health and Fitness blog has a new animated demonstration, this time on working your core by rotating your torso. This can also be done by using a pulley if you want more resistance than a stretch band is likely to provide. A variation that I am a fan of wood chops, where you take a high pulley and do a similar motion that is in the animated image, but you are pulling your arms and torso from the right shoulder to your left quad, and then the left shoulder to your right quad.