The Russian Twist will work into the obliques, transverse abdominis and hip-flexors. A really great all round core exercise.
Set yourself up with a kettlebell or dumbbell approximately 10cm away from your body on one side (ignore this step if using body weight). Position yourself so that your bum is on the floor, your legs are in front of you with an approximate 90 degree bend in the knee and heels are on the floor. You should be lent back such that the angle between your thighs and upper body is approximately 90 degrees.
Rotate your upper body following your hands as you bring them down beside you to pick up your weight. Carry the weight across your body and bring it down to the other side of your body so that it reaches a centimetre or two away from the floor, before bringing it back over to the other side.
If you are finding this easy you can progress this movement by lifting your feet up and off the floor. When doing this progression, try to keep your feet as still as possible.
This movement is all about control, so do not feel pressure to increase the speed of your rotation. Quality is more important than quantity!