The science: Human movement occurs on three different geometric planes:
1. The sagittal plane, for front-to-back and up-and-down movements
2. The frontal plane, for side-to-side movements
3. The transverse plane, for rotational movements
Most weight-lifting movements—the bench press, squat, curl, lunge, and chinup, to name a few—are performed on the sagittal plane; the balance of exercises—for instance, the lateral lunge and side bend—occur almost entirely on the frontal plane. This means that most men rarely train their bodies on the transverse plane, despite using rotation constantly in everyday life, as well as in every sport. Case in point: walking. It's subtle, but your hips rotate with every step; in fact, watch a sprinter from behind and you'll see that his hips rotate almost 90 degrees. By adding a rotational component to any exercise, you'll automatically work more muscle—since you'll fully engage your core, as well as the original target muscles—and simultaneously build a better-performing body.
Apply it: Simply twist your torso to the right or left in exercises such as the lunge, situp, and pushup. You can also rotate your hips during movements such as the reverse crunch.