Standing broomstick twists are another exercise that are ineffective and risky, especially for those with lower back problems. You perform broomstick twists by standing with a broomstick across the shoulders and twisting side to side. This exercise did not even make the top 13 of best abdominal exercises (http://www.acefitness.org/pressroom/246/american-council-on-exercise-ace-sponsored-study/), and puts stress on the ligaments and connective tissue of the spine. An even more dangerous variation is the bent over twist; you do the same movement bent forward from the spine, adding forward flexion, and increasing the pressure on the discs.
If you feel you need this exercise for a sport that requires rotational strength such as baseball or tennis, you can make it a little safer by keeping the movement controlled (avoid momentum) and slightly bending your knees. Avoid the bent over variation.