Fitness tip: Barbell anti-rotational exercises are great for core stability. I am working my entire core, particularly the obliques, and my glutes. The whole point of this is to resist rotation in my torso and hips. When the bar is moving to one side, the obliques on the opposite side of the body will be working big-time to resist rotation.
Here are 3 different options:
1) Standing single arm (tossing bar to make more advanced)
2) Standing double arm
3) Half kneeling single arm
1) Keep your elbow slightly bent but relax your arms as much as possible. Otherwise your arms will dominate and you won't feel your core at all. People will usually add too much weight and the end result is that they feel all upper body and no core. If you are doing this properly, the barbell alone will likely suffice.
2) If you are kneeling, make sure that your knee and toes are in line, and that your foot is stable. This will allow you to better utilize the glute (and I find VMO) of the front leg.
3) Keep your ribs tucked. If they are flaring, you will have less support from your anterior core and the muscles in your lower back, your hip flexors, TFL's, or adductors will take over.
4) The more your arm(s) travels laterally, the more challenging the exercise will become.
Being able to resist rotation (main function of the core muscles) will dramatically improve your performance in the gym, when you play a sport, or during your daily life, and will reduce your risk of injury.